BIO112.02 Laboratory Guide #3
DIVERSITY OF THE INVERTEBRATES
INTRODUCTION
The invertebrates are an extremely large and diverse group of organisms. They include animals that are of interest from economic and biomedical viewpoints as well as organisms that are just fascinating and of natural appeal. The diversity of the invertebrates may be organized into from 8 to over 30 extant phyla, most with numerous classes, orders, etc. It is important to keep the taxonomic placement of the organism under study in mind as you go through the exercises. In this way you will begin to learn the relationships among groups as you explore their morphology, anatomy, and life histories.
There is a very important difference between the evolution of animals and that of plants. Plant evolution is closely tied to the colonization of land and the slow radiation of progressively "higher" plants into ever drier and more inhospitable niches. It is characterized by the successive replacement of the nonvascular mosses first by the vascular ferns, then by seed-bearing gymnosperms, then finally by angiosperms as the dominant and most speciose group. In sharp contrast all of today's animal phyla (along with even more extinct ones) appeared in the oceans during a very short evolutionary burst some 650 million years ago, during the "Cambrian Explosion". Each phylum has radiated and diversified to a greater or lesser degree over the course of time, with most eventually becoming extinct. The invasion of the land allowed some of these phyla to diversify enormously, but no completely new body plans or life cycles were developed. At present, the mollusks, chordates, and especially arthropods seem to be enjoying the most success. One way of graphically visualizing this difference is to picture the evolutionary "tree" of the plant divisions as a climbing vine and that of the animal phyla as a very broad-based shrub.
In order to understand the nearly bewildering degree of variety among the invertebrate groups, you will need to master several key terms which refer to the basic, alternative designs of invertebrate bodies. These are listed here:
Diploblastic: being derived from two primordial germ cell layers, endoderm and ectoderm. (True mesoderm is missing in these organisms).
Triploblastic: being derived from three primordial germ cell layers, endoderm, ectoderm, and mesoderm.
Acoelomate: lacking a true body cavity. This term is typically restricted just to the triploblastic organisms.
Pseudocoelomate: having a body cavity which is not lined with mesoderm.
Coelomate: having a body cavity lined with mesoderm. The body cavity may be partitioned with septa, either bilaterally or segmentally.
Schizocoelous : having a true body cavity that forms from the splitting of solid blocks of mesoderm to open a new (novel) cavity.
Enterocoelous: having a true body cavity that forms from the outpocketing of a portion of the preexisting archenteron (primitive gut cavity).
After completing this laboratory you should be able to:
1) To outline and describe in appropriate terms the diversity of the invertebrate animals.
2) To describe the basic body plans of the major invertebrate phyla.
3) To group these phyla based on central features of the body plan and developmental patterns using both traditional and modern systematics.
4) To describe the major patterns of variation and taxonomic sub-groupings within these major phylum - down to at least the level of taxonomic Class.
5) To identify and classify representative animals from each phylum and class.
6) To describe the unique ecological features of each phylum and class.
Materials
Live, preserved, dried, and/or fossil specimens for each taxonomic group.
Poster guides to the diversity within each phylum.
Procedure
Specimens of each major invertebrate phylum and many major classes are on display. These specimens include preserved animals, concreted structures such as shells, and live animals.
1. Work through the displays of preserved specimens and microscopic slides.
a. A dichotomous key which outlines the major distinguishing features of each invertebrate group/class is provided below.
b. Following the key is additional information about each phylum, as well as a guide to lower taxa (subphylum, class) within each phylum. On the sample specimens, identify and examine all of the external structures written in bold print in this guide.
c. Additional information will be on display with the specimens.
d. Your textbook has additional information on these invertebrate phyla. The Biology Department has several invertebrate biology textbooks.
2. Your instructors have assembled as large a variety of live specimens as possible. Compare these live specimens with the preserved ones. Handle the live specimens with extreme care! Accompanying each live specimen will be additional information and instructions. Read through and follow these to gain additional insight into the structure, physiology, and behavior of theses animals.
3. Be able to describe the distinguishing features of each phylum of invertebrates. Within each phylum, be able to recognize the major subdivisions.
4. Be able to use the distinguishing features to reliably classify these sample invertebrates.
Study Suggestions
1. Make detailed sketches and notes on specimens. This will help you to look at the specimens more closely, as well as to help you study later.
2. Plan to come view the specimens once or twice more before the lab test. Test yourself by attempting to identify the specimens as accurately as possible by their common names, as well as to classify them without first looking at their labels.
3. The words in bold print in the extended guide below are words you should know and/or structures you should be able to identify or describe.
A Dichotomous Key to the Adult Invertebrates
Note: This key is based, in some cases, on secondarily derived characteristics of adult animals. Therefore, it does NOT always follow phylogenetic relationships. It might be useful for you to construct an alternative key which does reflect actually phylogeny, both to help you understand the true systematics of the invertebrates, and to see why a pragmatic, balanced key might not follow these.
1. asymmetric, radial, or pentaradial body
2. asymmetrical body; sessile, filter-feeding adult
3. lacking true tissues or nervous system Ph. PORIFERA
3. coelomate; pharynx with gill slits Ph. CHORDATA; SubPh. Urochordata
2. radially or pentaradially symmetric body
3. radial symmetry, bag-like body; diploblastic
4. stinging cells on tentacles Ph. CNIDARIA
4. ciliated combs, adhesive cells on tentacles Ph. CTENOPHORA
3. pentaradial body; triploblastic; enterocoelomate Ph. ECHINODERMATA
1. bilaterally symmetrical body; may have radially arranged limbs
2. unsegmented body
3. acoelomate, body dorso-ventrally flattened, blind digestive tract
Ph. PLATYHELMINTHES; Cls. Turbellaria , Trematoda
3. pseudocoelomate or coelomate; two-ended digestive tract
4. pseudocoelomate
5. body elongated and worm-like Ph. NEMATODA
5. microscopic; mouth bearing ciliated wheel-organs Ph. ROTIFERA
4. coelomate
5. dorso-ventrally paired shells; enterocoelomatePh. BRACHIOPODA
5. shell single, laterally paired, internal or absent; schizocoelomate;may show visceral torsion or radial limbs Ph. MOLLUSCA
2. internally and/or externally segmented body; some segments may be fused
3. acoelomate; body dorso-ventrally flattened; no digestive system
Ph. PLATYHELMINTHES; Cl. Cestoidea
3. schizocoelomate or enterocoelomate
4. schizocoelomate
5. open circulation
6. jointed appendages Ph. ARTHROPODA
6. non-jointed appendages Ph. ONYCHOPHORA
5. closed circulation Ph. ANNELIDA
4. enterocoelomate
5. dorsal notochord Ph CHORDATA; Subph. Cephalochordata
5. no notochord; prominent everted proboscisPh. HEMICHORDATA
GROUP I. PARAZOA
Members of Subkingdom Parazoa lack both the true tissue-level organization and the nervous system of the other animals. Sponges are the only extant parazoans.
PHYLUM PORIFERA (SPONGES)
The sponges, aptly named as the Phylum Porifera, or "pore-bearers", are poorly organized diploblastic animals that are sometimes referred to as little more than colonies of cells. Sponges are highly dependent upon a few limited types of cells, each working somewhat independently, yet still contributing to the overall good of the sponge as a whole, hence the colonial nature of this simple organism. Sponges function by using specialized flagellated cells (choanocytes) to create a water current that passes through the wall, bringing in essential materials from the surrounding water and carrying away waste products. The most common error in understanding sponge design is to interpret the large opening at the top of most sponges as a "mouth". This opening, called the osculum, is actually the exit of the water current, not the entry. The correct path of water flow is into the sponge through the typically microscopic pores along the walls and out through the osculum. We will examine three classes within this phylum.
Class Calcarea
The sponges in this group have spicules composed of calcium carbonate (CaCO3). They are typically very small in size and may appear as a thick fuzzy layer encrusting undersea rocks, shells, or coral.
Examine the prepared slides of Leucosolenia, both longitudinal and cross sections. Note the simple structure of this sponge. Note the spicules, the skeletal elements of sponges. They should appear as glassy, 3-rayed stars.
Examine the prepared slides of Scypha (=Grantia), a more complex sponge. Note that the walls are convoluted (folded).
Class Demospongia
This class is by far the largest of all of the classes of sponges and includes the most complex types of sponges. Most larger marine sponges, and all freshwater sponges belong to this class. The demosponges have a skeletal structure composed of spicules made of silica and/or spongin, a soft, proteinaceous material that forms a spongy matrix. Sponges with a skeleton made only of spongin are called the “bath sponges”.
Examine the specimens of sponges of the class Demospongia on display. Locate the major features of the typical sponge morphology for each.
Examine the live sponges on display. Locate the pores and osculum on each.
Class Hexactinellida
The name of this class is derived from the fact that the spicules typically have six points. Some of the spicules fuse to form a lattice-like siliceous skeleton, and therefore are also called glass sponges. Examine the representative of this class on demonstration, commonly called the “Venus’s flower basket” .
EUMETAZOA (or METAZOA)
True metazoans have cells organized into specialized tissues and have nervous systems for coordinating both physiology and behavior. A major classical taxonomic distinction is between the protostomes, in which the blastopore becomes the mouth and the deuterostomes, in which the blastopore becomes the anus. The first several groups of phyla which we will be studying are all protostomic.
GROUP II. RADIATA
These phyla have radially symmetrical, bag-like, diploblastic bodies and simple neural networks.
PHYLUM CNIDARIA (STINGING-THREAD ANIMALS)
The Phylum Cnidaria derives its name from the everting stinging cells called cnidocytes, which shoot out miniature barbed harpoons called nematocysts The group has achieved a greater level of structural complexity than that of the sponges; true tissues are present and a rudimentary organ-level organization can be seen. However, these animals are diploblastic, meaning they lack the middle germ layer mesoderm. The three classes that we will examine today in lab differ from each other in several ways, including life history patterns.
As a phylum, the group displays a life cycle with metagenesis, sometimes also referred to as alternation of generations. Unlike plants, both generations are diploid - the alternation is between asexually-reproducing polyp and sexually-reproducing medusa (jellyfish) forms. This information is nicely presented in your textbook and will be discussed in lecture. In lab, you will notice that in general, the structural organization of the classes becomes more complex as you move through each group in the order presented below. However, the classes tend to be somewhat heterogeneous with regard to their membership so careful study is required to learn the class organization correctly.
Class Hydrozoa (hydroids and fire-corals)
The hydrozoans are cnidarians with simple gastrovascular cavities and have a metagenetic life cycle wherein the polyp phase is dominant. The marine hydroid Obelia is a typical representative. The more familiar fresh-water Hydra is atypical in that it totally lacks a sexually-reproducing medusa in its life cycle.
Observe prepared whole mounts of Obelia, a polymorphic, colonial hydrozoan. Locate the dactylozooids, the feeding polyps, and the gonozooids, the reproductive polyps. Locate the shared gastrovascular cavity and the perisarc, a clear covering secreted by the epidermis. Review the life cycle of Obelia. A good diagrammatic illustration of this life cycle is found in your textbook.
Examine the live specimens and prepared slides of the monomorphic hydroid Hydra that show the development of a bud, an asexually reproduced clone of the adult organism.
Observe the preserved specimen of Physalia, the Portuguese “Man–o–War.” Physalia is another colonial hydrozoan. However, it is atypical for hydrozoans in that it has no sessile polypoid stage. Locate the long feeding tentacles, short reproductive tentacles and the pneumatophore (float) which supports the colony at the surface of the water.
Class Scyphozoa (true jellyfish)
Scyphozoans are cnidarians with a subdivided gastrovascular cavity and a metagenetic life cycle wherein the medusoid phase is dominant.
Observe the preserved specimen of the jellyfish. Locate the mouth, oral tentacles, and umbrella (bell). Note the extremely thickened mesoglea, the so-called jelly. Locate the gastrovascular cavity near the top of the umbrella. The fluffy structures extending into the cavity are the gonads. Jellyfish can be either monoecious or dioecious.
Observe the live specimens of the "upside-down" jellyfish Cassiopeia. This jellyfish lives among the roots of tropical mangrove trees at the edge of the ocean. Its habit of lying upside-down exposes symbiotic, photosynthetic algae around the mouth cavity to sunlight. The jellyfish draws much of its nourishment from these algae. How does this animal propel itself?
Class Anthozoa (sea anemones and corals)
Anthozoans are cnidarians with a complex gastrovascular cavity, partitioned with internal sheets or septae. There is no medusoid phase for this class, so alternation of generations does not occur.
Examine the demonstration specimen of Metridium, a large solitary sea anemone. Locate the tentacular crown, body stalk and pedal disk. Follow the mouth down the pharynx to the gastrovascular cavity. Notice that the cavity is divided by a series of septa. What function do these septa perform? Is there an anus? Is this a colony or one individual?
Observe the demonstrations of representative hard corals. These hard or stony corals have cup-like depressions called calyxes that house the polyps. Notice the septae that correspond to the septae found in the gastrovascular cavity of the polyp. They provide additional surface area affording the polyp a more secure seat. The coral skeleton itself is made of calcium carbonate (CaCO3) that is secreted by the undersurface of the polyps.
Observe the demonstration of soft corals: sea fans and sea whips. Locate the tiny calyxes similar to those of the hard corals. Soft corals have two skeletons: a central horny or wood–like axis and a matrix of spicules surrounding this axis.
Observe the live coral on display under a dissecting microscope. Notice the tiny living polyp in each coral calyx.
PHYLUM CTENOPHORA (COMB JELLIES)
The comb jellies used to be classified with the cnidarians as the Phylum Coelenterata, meaning "bag animal". Like the cnidarians they are diploblastic, with a bag-like, radially symmetrical architecture and a simple, distributed, web-like nervous system. The distinguishing features of the ctenophorans are 1) eight comb-like rows of cilia which beat rhythmically to propel the animal and 2) a pair of tentacles armed with adhesive structures called colloblasts.
Observe the live comb-jellies on display. Look for the combs of beating cilia. Is the ciliary beating pattern synchronized or organized in any manner that you can identify? How does this animal propel itself?
GROUP III BILATERIA; ACOELOMATES
These phyla are bilaterally symmetrical and triploblastic. The body interior is filled with solid, spongy mesenchyme tissue and lacks a body coelom or internal body cavity. Flatworms are the only representatives of this group which we will study.
PHYLUM PLATYHELMINTHES (FLATWORMS)
The Platyhelminthes are quite literally the “flat” (=platy) “worms” (=helminth). These are dorsoventrally compressed, triploblastic acoelomates. Each class is quite unique. The free-living members of Class Turbellaria are most representative of the ancestral condition; the other three classes show a wide variety of specializations that go along with the evolutionary move to a parasitic way of life. Most, but not all, trematodes are hermaphroditic (monoecious) - containing both male and female sexual organs and reproductive tracts.
Class Turbellaria (free-living flatworms)
Examine the prepared slides of Planaria stained to show the digestive system. Locate the mouth, pharynx (proboscis) and the branched intestine. What advantage does this system show as compared to a simple sack intestine? Find the light-sensitive “eyes”.
Examine a slide with a cross-section of a planarian. Find the intestine and the mesenchyme, a loose aggregation of cells that fills the space between the outer body wall and internal organs. Note the total absence of a body cavity.
Observe the live planaria on display. Conduct a simple experiment to determine how they react to light. Are they positively or negatively phototaxic? How does this relate to their role as scavengers and detritus-feeders?
Classes Trematoda (digenetic flukes) and Monogenea (monogenetic flukes)
Flukes are parasites, some of which have complex life cycles involving two or more hosts. Monogenic flukes are principally ectoparasites with vertebrate hosts. They cling to well vascularized animal surfaces such as fish gills, mouth cavities, and the peripheral urogenital system, using an enlarged opisthaptor . The opisthaptor may be armed with multiple suckers and/or hooks. As the name implies monogenetic trematodes have a simple life cycle progressing from a parasitic adult to eggs to free-swimming larvae and back to parasitic adults.
Digenetic flukes are endoparasites whose life cycles involve more than one host. Adult digenetic flukes typically inhabit vertebrate "definitive" hosts and live in locations ranging from the circulatory system to the bile ducts of the liver. Eggs are passed to the outside and develop into free-swimming larvae. These infest an invertebrate "intermediate host”, generally a fresh-water snail or marine annelid. In the invertebrate host they form cysts in muscle, brain, heart, lungs, gonads, or other highly vascular tissue. These cysts may passed to the vertebrate host when it ingests the infected invertebrate or they may hatch into free-swimming larvae which burrow into the host. Many flukes are parasitic on humans and domesticated animals, hence they are of medical importance to ourselves and many of the animals we raise for food production.
Examine the whole mount slides of the human liver fluke Opisthorchis sinensis (= Clonorchis) and the larger sheep liver fluke Fasciola hepaticum. Locate both the both the oral and ventral suckers. Trace the digestive system from the anterior sucker surrounding the mouth, to the pharynx and branched intestine. Trace both the male and female reproductive tracts.
Examine the slide containing large adult flukes and two smaller larval forms: rediae which infest the invertebrate host and free-swimming, tailed cercaria which burrow into the vertebrate host.
Observe the slides of the dioecious, sexually dimorphic, human blood fluke Schistosoma mansoni. The male is the smaller animal clasped in copulo in the long ventral folds of the female.
Class Cestoidea (tapeworms)
The cestodes are a group of intestinal endoparasites of both vertebrate and invertebrate hosts. As with the flukes, they are important in relation to human and farm animal health. Tapeworms are divided into distinct segments called proglottids. Each proglottid contains a complete set of both male and female reproductive organs. New proglottids form in the neck region, while the oldest and most mature gravid (egg-producing) proglottids are found at the posterior end of the body.
Examine the prepared slides of the tapeworm Taenia pisiformis, an intestinal parasite of mammals including humans. Find the head or scolex. What types of attachment devices are found on the scolex? Find the individual segments or proglottids. Locate an immature proglottid near the anterior end, one in which none of the internal organs have clearly formed. Now find a mature proglottid near the middle of the animal. The structures you see are almost all for the purpose of sexual reproduction. Finally, find a gravid proglottid near the posterior end of the animal. Locate the egg bearing capsules that fill the swollen uterus. Notice that there is no digestive tract in this animal. How does it nourish itself?
Tapeworms are also digenetic, although both the intermediate host and the final host may be vertebrates. As discussed in class, the larval forms of some tapeworms form fluid-filled hydatid cysts in the internal organs of the intermediate host. These cysts can be quite large and debilitating. The larval form is generally passed to the final host when the intermediate host is killed and eaten.
GROUP IV BILATERIA; PSEUDOCOELOMATES
These phyla are bilaterally symmetrical and triploblastic. They have an internal coelom, but it is not lined with mesoderm.
PHYLUM ROTIFERA (WHEEL ANIMALS)
Rotifers are microscopically tiny, multicellular organisms. They get their name from the wheel-shaped, ciliated jaws which sweep particulate food into the mouth. In spite of their small size they are anatomically fairly complex. Their digestive system is double-ended and they have an unlined internal body cavity. Many reproduce by parthenogenesis; that is all individuals are females which develop from and produce diploid, unfertilized eggs.
Observe the live rotifers on display. Look especially for the rapidly-beating cilia of the circumoral wheel organs.
PHYLUM NEMATODA (UNSEGMENTED ROUND WORMS)
The nematodes comprise a large number of taxa that include free-living forms as well as both plant and animal parasites. This phylum is a very successful group and their existence contributes to the welfare of humans in both positive and negative ways.
Obtain a whole specimen of Ascaris for dissection. These worms are intestinal parasites of mammals including humans. This species demonstrates sexual dimorphism with the males having a hooked posterior and bearing sharp spines called copulatory spicules near the posterior tip. What is the sex of your specimen? Carefully make a longitudinal cut through the cuticle and body musculature. Extend the cut along the entire length of the animal and pin it open to expose the internal organs. Find the short pharynx and long, strap-like intestine that terminates at the anus. In both the male and female, the reproductive system is long and tubular. In females, the system is paired, while in males it is singular. Try to determine whether the specimen you have is male or female. Then look at a specimen of the opposite sex.
Observe the live sample of vinegar eels under a dissecting microscope. These animals have longitudinal muscles running the length of the animal, but no radially-arranged muscles around the circumference. Observe their wriggling. Which of the following motions are present: lateral flexing, coiling, undulating waves of contraction which run from one end of the animal to the other? It may be necessary to put a few of these animals in a depression slide and slow them down with some Proto-Slo.
GROUP V BILATERIA; SCHIZOCOELOMATES
These phyla are bilaterally symmetrical and triploblastic. They have an internal coelom lined with mesoderm which forms by splitting solid blocks of mesoderm.
PHYLUM MOLLUSCA (MOLLUSKS)
The mollusks are the second largest phylum of animals behind the Arthropoda. Included in the Mollusca are many organisms found commonly along the sea coast, in the garden, and in fresh water streams and ponds. The group is highly diverse and successful with numerous organisms that display unusual, often bizarre, adaptations that have allowed them to compete successfully in an evolutionary sense. Most are dioecious, having separate male and female individuals.
Mollusks are characterized by an unsegmented muscular body mass which includes a foot, a visceral mass containing the organs, and a specialized fold called the mantle. The mantle houses a mantle cavity containing the gills and is connected to the outside world via tube-like incurrent and excurrent siphons. In some groups the mantle secretes a calcareous shell. The mouth has a distinctive file-like structure called the radula.
We will study four major classes of molluscs in this lab.
Class Polyplacophora (chitons)
Chitons are marine animals with a relatively simple body plan, probably similar to that of the common molluscan ancestor. The body is covered with a shell compose of eight plates.
Examine both the preserved chiton specimens and the live chitons on display. Try to dislodge the live chiton (no prying implements please!). The chiton's foot serves as an effective suction cup to hold it to the surface upon which it grazes.
Class Gastropoda (snails and slugs)
The gastropods (literally "stomach-foots") inhabit marine, freshwater, and terrestrial habitats. Both shelled (snails, whelks, etc.) and unshelled (slugs and nudibranchs) forms exist. They are characterized by a distinct head containing cerebral neural ganglia and a pair of eyes on extensible stalks.
The shelled forms of gastropods strongly exhibit a secondary developmental pattern called torsion. This process rotates the anus up over the head and results in a spiraled, somewhat asymmetrical animal. Examine the diversity of form in shells of the marine gastropods on display here. Both planospiral and helically coiled types are present.
Examine the live terrestrial snails Helix on display. What sorts of stimuli cause the head to retract into the shell? The path of a snail can often be traced by the trail of slime which it leaves behind. In terrestrial snails this slime also forms an effective plug which seals the animal in its shell during dry periods and prevents desiccation.
Class Bivalvia or Pelecypoda (clams, mussels, and oysters)
Bivalves have two shells which are connected by a dorsal hinge. Most are relatively sessile filter-feeders, extracting plankton from water which is pumped through the gills and out of the siphon. Some species are ectoparasites of fish, especially of the fins and gills. When disturbed many clams can propel themselves for short distances backwards by repeatedly snapping the shells together and/or forcefully expelling water through the excurrent siphon.
Examine the preserved specimen of a clam (Mercenaria). Locate the two adductor muscles, which keep the clam closed. Locate the sheet-like mantle fold, the secretory organ that produces all three shell layers. It can be found adhering to the inner surface of the shell. Find the lamellar gills and the muscular foot. What functions do each of these structures perform? Find the major components of the shell including the umbo (center of growth), the hinge line, the elastic ligament which serves to open the shells, and the adductor muscle scars which are cuplike depressions in the inside of the shell that mark the positions of the adductor muscles.
Closely examine the live flame scallop, a marine bivalve. Notice the bright red tentacles extending from the opened shell. Look closely for the light-sensitive eyes at the base of the tentacles. What is the response to the animal to a gentle poke in the region of the tentacles or a tap on the shell?
Class Cephalopoda (cuttlefish, nautilus, squids, and octopi)
The cephalopods, as the name suggests, have the most elaborate, centralized, and cephalized nervous systems of all of the invertebrates. They also exhibit a radial organization of the arms and the associated peripheral nervous system.
Observe the demonstration specimens of the squid Loligo . Locate the muscular sac-like mantle. Find the head with the circumoral ring of arms and tentacles and well developed paired eyes. The arms are relatively short and have suckers along their entire length. The longer single pair of tentacles have suckers located only at the expanded distal portion called the club. Find the excurrent siphon on the ventral surface of the body at the junction between the head and mantle. The funnel is used to direct the flow of expelled water used for “jet propulsion.”
Observe the demonstration of another cephalopod, Octopus. How does this animal differ from Loligo? List several features.
Observe the chambered nautilus shell. How does this shell differ from that of a typical gastropod?
Finally, observe the "cuttlebone" from a cuttlefish. Is this an external or an internal shell?
PHYLUM ANNELIDA (SEGMENTED ROUNDWORMS)
The Phylum Annelida includes the segmented worms. In addition to metamerism, the serial repetition of body parts, the annelids display a closed circulatory system, a spacious coelom (schizocoel) that acts as a hydrostatic skeleton, a high degree of cephalization, and a well developed, muscular body wall. The group is triploblastic. The annelids are divided into three classes; Polychaeta, Oligochaeta, and Hirudinea.
Class Polychaeta
The polychaetes bear paired lateral appendages called parapodia. They have a well–developed head that typically bears a variety of sensory structures. The polychaetes are dioecious but lack discrete gonads. The gametes arise directly from the peritoneum, the lining of the coelom. Development is indirect, i. e. a larva is produced. There is no clitellum and copulation does not take place. Instead broadcast spawning is followed by external fertilization. The group is marine.
Observe the specimens of Nereis, the clamworm or sandworm. Locate the paired, segmentally arranged appendages, the parapodia. Examine the well developed head region composed of two modified segments. There are a variety of sensory structures located on the head including pigmented eyes and elongated tentacles. Locate these structures. Also locate the large, paired jaws (mandibles) by carefully manipulating a probe in the mouth area.
Class Oligochaeta
In future labs, you will study features of the digestive, circulatory, excretory and reproductive systems of the earthworm (Lumbricus).
For now examine closely the live specimens on display. What are some of the major differences between this animal and Nereis? Which do you feel is more typical of the phylum as a whole. Why?
Class Hirudinea
The class Hirudinea includes the leeches. Although some leeches are predators which feed on small microcrustaceans, most are ectoparasites that feed on the blood or lymph of a host organism. The pharynx is modified as a pumping organ to suck in these juices. The mouth is surrounded by an adhesive sucker and a second sucker may be located near the posterior end of the body. Why? One anatomical difference between the leeches and the other annelids is that leeches lack complete septae internally separating adjacent body segments.
Examine the external morphology of the leech placed out on demonstration. Note that it lacks both appendages and setae. Find the anterior and posterior suckers. What is the function of each?
Examine the live leech specimens. Carefully dislodge one from the side or bottom of the aquarium and watch closely as it swims. What is different about its motions from what you observed in the nematode vinegar eels?
PHYLUM ARTHROPODA (JOINTED-LEGGED ANIMALS)
The Phylum Arthropoda is the largest and most successful group of animals on earth. In fact, this phylum includes about 3/4 of all species that inhabit this planet. They have successfully conquered both aquatic and terrestrial areas and have mastered flight as well.
The group includes the horseshoe crabs, spiders, shrimp, lobsters, barnacles and insects.
The basic features of the phylum include an exoskeleton with paired, jointed appendages; a body divided into three main regions called tagmata (head, abdomen, and thorax); a reduced coelom; complete digestive system; open circulatory system; and a high degree of cephalization with a broad array of complex sense organs. Arthropods undergo a process called molting as they grow. Many arthropods also pass through one or more distinct, structurally different stages, a process called metamorphosis.
Many arthropods have very complex patterns of behavior. Some have developed a social community structure and polymorphism. Due to the size and diversity of this group, derivations and modifications of the basic body plan are common.
Subphylum Chelicerata
The chelicerates include a diverse assemblage of organisms including spiders, mites, ticks, scorpions, and the horseshoe crab, which in reality is not a crab at all. All these organisms lack antennae and mandibles. The first pair of appendages is called the chelicera. The body is generally divided into two distinct multi-segmental tagmata: and anterior cephalothorax and a posterior abdomen.
Examine the demonstration specimens and live specimens of the horseshoe crab, Limulus polyphemus, a member of the Class Merostomata and a so-called "living fossil" from the Ordovician Period. Locate the carapace with its dorsal compound eyes. Flip the specimen over and count the seven pairs of jointed legs. Locate the book gills and telson (caudal spine) on the abdomen. How do the book gills get their name?
Examine the preserved and live specimens of a typical member of the Class Arachnida, the American tarantula. Count the six pairs of legs; from front-to-back the chelicera (modified to form venomous fangs), the pedipalps, and four pairs of walking legs. This is probably best done on the preserved specimen! Note the multiple compound eyes on the cephalothorax and the spinnerets at the tip of the abdomen. At some point in the class your instructor will feed a cricket to the tarantula so that you can watch this predator in action.
Examine the preserved specimen of the scorpion, another terrestrial arachnid. Notice that the second pair of legs, the pedipalps are modified into large claws. The caudal abdominal segments are modified into a tail tipped with a venomous stinger.
Subphylum Crustacea
The crustaceans are a large and successful group of organisms that include many common animals; indeed many that we consider good to eat such as the shrimp, crabs, lobsters, etc. In addition this group also includes the barnacles (once considered to be in the Phylum Mollusca), pill bugs, and water fleas. Crustaceans have two pairs of antennae and multiple pairs of biramous appendages (= "two-branched") on both the cephalothorax and the abdomen. At least three pairs of these most rostral legs are modified feeding appendages. Both crustaceans as well as uniramians (the next group) have mandibles rather than chelicerae.
Crustaceans range in size from microscopic to several kilograms. Major groups include the decapods (crayfish, lobsters, shrimp, and crabs), branchiopods (fairy shrimp and daphnia), copepods, ostracods, isopods (pill bugs), and amphipods (water fleas).
Obtain a live specimen of the crayfish Procambarus for examination. Observe that the body is divided into two regions, the cephalothorax and the abdomen. Locate the long antennae and the stalked eyes attached to the head. Most crustaceans have a shell-like chitinous carapace covering most of the cephalothorax. The point of the carapace extending anteriorly between the eyes is called the rostrum. Note that the mouth is surrounded by a series of specially modified, serially arranged mouth parts. The pair closest to the mouth are the mandibles. Locate the large pincers or chelae on the anterior set of walking legs. Locate the abdominal legs or swimmerets. What is their function? Locate the fan-like tail of the body called the telson. The anus is located ventrally at the base of the telson. Tap on the tail of the crayfish. It may exhibit a tail-flip, which is an extremely fast, stereotyped, escape maneuver.
Examine the live specimens of the common fiddler crab. Which crabs are the males? If crab morphology confuses you then grab one and flip it over. You will find the abdomen folded up under the cephalothorax.
Examine the live representatives of the smaller branchiopod crustaceans, including brine shrimp and Daphnia. What is the response of brine shrimp to light: i.e. are they positively or negatively phototaxic? How does this relate to their ecological role as plankton-feeders?
One rather deviant group is the Class Cirripeda, a.k.a the barnacles. Observe the live specimens of barnacles. The small fan-like appendages are the legs, which function to sweep in particulate food. If a barnacle doesn't look like a crustacean to you, think of it this way: a barnacle is nothing more than a small crustacean sitting on his head, with his carapace pulled in around him, and waving his feet out in the water.
Subphylum Uniramia or Mandibulata
Uniramians are characterized by unbranched legs and well-developed mandibles surrounding the mouth. The uniramians include many organisms that we might consider pests (stinging insects) as well as many others that are critically important to our way of life and general well-being (insect pollinators).
Carefully examine the preserved centipedes (Class Chilopoda) and the live millipede (Class Diplopoda). What are some of the major structural and ecological differences between these two classes?
Adult insects (Class Insecta) are characterized by a segmented body divided into three tagmata or parts; head, thorax, and abdomen. The head contains compound eyes, antennae, and a mouth with well-developed mandibles. The thorax contains six pairs of legs and two pairs of wings (in most insect orders). Respiration is through tube-like tracheae which open onto the abdominal surface through spiracles.
Examine the demonstrations of several orders within the Class Insecta. Which insects exhibit complete metamorphosis and which exhibit incomplete metamorphosis? Be able to trace the steps in both complete and incomplete metamorphosis, including the juvenile forms in each.
Examine the live Madagascar "hissing cockroach." Poke it to discover how it gets its name. The sound you hear is produced by air being forced out through the spiracles.
GROUP VI BILATERALIA; ENTEROCOELOMATES
These phyla are also bilaterally symmetrical and triploblastic. They have an internal coelom lined with mesoderm which forms from an outpocketing of the embryonic gut, the archenteron. Unlike the other eumetazoan phyla which you have studied, the enterocoelomates are deuterostomic, meaning that the blastopore eventually becomes the anus of the adult animal.
PHYLUM ECHINODERMATA (SPINY-SKINNED ANIMALS)
The Echinodermata is the most speciose and diverse extant deuterostome phylum of invertebrates. The entirely marine group includes the familiar starfish, sea urchins, sand dollars, brittlestars, sea cucumbers, and sea lilies. The echinoderms are pentaradially symmetrical animals with calcareous endoskeletons. The individual skeletal elements called ossicles may be variously modified or fused into a sack-like test. A unique feature of the echinoderms is the water vascular system which plays a vital role in locomotion, respiration, food acquisition and sensory perception.
Class Asteroidea (sea stars)
The asteroids are the common starfishes. They are characterized by arms which are not distinctly separated from the central disk and which have an open ambulacral groove on the under surface.
Observe the demonstration specimen of Asterias. Externally, locate the central disk and the arms. Find the oral (mouth bearing) and aboral (non–mouth bearing) surfaces. Find the madreporite and anus on the aboral surface. What is the function of each of these? On the oral surface locate the mouth, the open ambulacral grooves that run down each arm, and tube feet that extend out of the ambulacral grooves.
Remove a living orange sea star from the aquarium and examine it closely under a dissecting microscope. Find the light-sensitive oceli at the end of each arm. If you look closely at the dorsal skin you may be able to see the tiny pincer-like pedicellaria which keep the surface of starfish free of debris and ectoparasites. Return the sea star to a large finger bowl full of seawater, but place it upside down. How does it right itself? (This process may take several minutes). How do the tube-feet function in this process?
Class Ophiuroidea (brittlestars and basket stars)
The ophiuroids are easily distinguished from the asteroids (true starfishes) by the clear separation of the arms from the central disk and the absence of ambulacral grooves.
Observe the live brittle stars associated with the small sponge on display.
Class Echinoidea (sea urchins and sand dollars)
The echinoids include the sea urchins and sand dollars. The ossicles are fused to form a rigid test.
Obtain a specimen of a sea urchin. Notice that the entire test is covered with spines. Locate the mouth in the center of the oral surface. Around the mouth is located a flexible sheet of tissue, the peristomial membrane.
Examine the hard tests of sand dollars and sea urchins. The five-pointed (pentaradial) architecture is readily apparent in these. Find the complex jaw apparatus called Aristotle’s lantern in the plastic sea urchin mount.
Observe the live sea urchin specimens. How do the tube-feet and spines cooperate in moving the animal along the substrate?
Class Crinoidea (sea lilies)
At first glance sea lilies may also not look like the other echinoderms. Unlike the other echinoderms the oral surface is oriented upwards. Five feathery arms, each with several branches, extend upwards from a central calyx or body. Multiple tube feet used in locomotion and in suspension-feeding radiate out from pinnules, which in turn extend out from the arm branches.
Examine the preserved sea lily specimens to familiarize yourself with the basic body orientation and confirm the pentaradial architecture.
Class Holothuroidea (sea cucumbers)
The sea cucumbers are unusual organisms and somewhat atypical echinoderms. The body is expanded along the oral-aboral axis and the organism lies on its side on the bottom the sea.
Examine the preserved and live specimens of sea cucumbers. The oral end has a crown of short tentacles surrounding the mouth. The pentaradial architecture is expressed by the five rows of short tube feet running the length of the animal.
PHYLUM HEMICHORDATA (ACORN WORMS)
The hemichordates exhibit pharyngeal gill slits, a trait they share with the much more specious and diverse chordates. They are morphologically divided into an anterior proboscis, and middle collar surrounding the mouth, and a posterior body. Most hemichordates are tube-dwelling filter-feeders. Examine the preserved acorn worm specimens to identify these structures.
PHYLUM CHORDATA (ANIMALS WITH A NOTOCHORD)
The chordates are characterized by the presence of four structures, at least in the larval forms: pharyngeal gill slits, a notochord, a dorsal hollow nerve chord, and a post-anal tail. This group includes both invertebrates and vertebrates. Unlike the vertebrates, the invertebrate chordates exhibit virtually no cephalization (centralization of nervous tissue in the head). The two invertebrate chordate subphyla are structured rather differently, but both are filter-feeders.
Subphylum Urochordata (tunicates or sea-squirts)
Free-swimming tunicate larvae ("ascidian tadpoles") exhibit all four of the chordate characteristics. However, adult tunicates are sessile bag-like animals which retain only the pharyngeal gill-slits.
Examine preserved specimens of both larval and adult tunicates. In the larva try to identify the notochord, nerve chord, pharyngeal gill slits, and tail. In the adult identify the pharyngeal basket, incurrent mouth, and excurrent atrial siphon.
Subphylum Cephalochordata (sea lancelets)
Free-swimming adult sea lancelets exhibit all four chordate characteristics.
Examine the preserved specimen of the lancelet Branchiostoma (formerly Amphioxus) under a dissecting microscope. Identify the notochord, pharyngeal gill slits, and mouth, surrounded by buccal cirri. Notice also that the muscle in this animal is clearly divided into blocks, revealing the segmented body plan characteristic of both invertebrate and vertebrate chordates.
Examine the live sea lancelet. Gently induce it to swim. Notice that its swimming is much more efficient than what you saw in either the vinegar eels (nematode) or leech (annelid). What structure allows this efficient transfer of force from the muscles to the water?