MOLLUSCA AND ECHINODERMATA

Introduction

The two phyla to be examined in this exercise are considered by most biologists to represent the highest evolutionary development among the invertebrates. There is substantial evidence that the mollusks descended from annelid ancestors, while the evolutionary tree of the echinoderms is less well defined. In their embryonic development, the echinoderms have enough in common with the chordates that we must consider them our very distant relatives.

Phylum Mollusca

Phylum Mollusca is made up primarily of marine and freshwater organisms with a long history on the earth. While they are mostly bottom dwellers, the mollusks occupy a wide variety of habitats and exhibit a diversity of life styles. This has led to Mollusca having more species than any phylum except the arthropods. Characteristics common to most members of the phylum are:

A. The internal organs are confined to a visceral mass, which is enclosed in the mantle.

B. The organism is supported and/or protected by a shell secreted by the mantle.

C. A prominent organ is the muscular foot, which has been modified to perform several functions within the phylum.

D. In the mouth is a radula, a sort of spiked tongue, which is used in feeding.

Although there are several minor taxonomic groups within Phylum Mollusca, the following classes are considered most significant:

1. The bivalves include some of the most familiar invertebrate animals. Among them are clams, oysters and scallops. These filter-feeders lack a head or radula. Oysters and scallops also lack the typical foot, but most bivalves have a very prominent hatchet-shaped foot with which they burrow into the bottom. All, however, have a two-part, hinged shell, which encloses most of the visceral mass. One interesting member of the class, the shipworm, has small shells, which are used to bore into submerged timbers. In the past they were greatly feared by sailors of wooden ships and even today they cause much damage to peers, wharves, etc.

ACTIVITIES

a. Consider the collection of bivalved shells. If you see any of these again, can you give their kingdom, phylum and class? Do not continue until your answer is “yes”!

2. The gastropods probably show more diversity than any group of mollusks. Snails, slugs and nudibranchs (sea slugs) are members of this class. Gastropods may be herbivorous, carnivorous or omnivorous. The cone shells are fearsome predators, which utilize a poisonous stinger to kill their prey, usually other snails. The gastropods are the only mollusks to evolve land-dwelling species. A spiraled shell characterizes all the snails. Their digestive systems have become twisted to allow them to expel feces out the one opening of the shell. Gastropods have a well-developed head with functional eyes, tactile tentacles and a radula. Slugs are similar to snails, but they typically lack a shell and have not undergone torsion. Like most gastropods, they move about on a flat, muscular ventral foot. The nudibranchs are large, brightly colored marine animals. They are active swimmers and feed on jellyfish and other cnidarians.

ACTIVITIES

b. If live snails are available, place one in a finger bowl with some aquarium water and observe it for a few minutes. Note the single-valved, spiraled shell. Can you see the head with its tentacles? Note the smooth, gliding motion of the snail as it moves on its flat foot. Carefully touch the snail’s head and watch it retract into its shell. A photograph of a live snail is shown in Figure 98c of your Perry and Morton Photo Atlas.

c. Examine the collection of gastropod shells. What common features may be observed in all the shells? Be able to identify each as an animal, a mollusk and a snail.

3. The chitons are perhaps the most primitive of the mollusks. They have a shell made of eight overlapping plates. The mantle surrounds the flat, ventral foot, making the flexible chiton a living suction cup! Chitons inhabit the smooth rocks of the surf zone, quietly rasping algae from the rocks with the radula, held tight against the power of the waves by their unique body design.

ACTIVITIES

d. Examine the chiton provided and identify its shell plates, mantle and.

4. The cephalopods represent the highest evolutionary development among the Mollusca. In this class, the foot has become modified into a group of suckered tentacles, which surround the mouth. In most cephalopods, the mouth is a horny, parrot-like beak with a radula. These animals have the most highly developed eyes of all the invertebrate groups. Probably the most numerous of the cephalopods are the squids. These creatures have a streamlined body, modified for swimming, with a much-reduced internal shell. Like other cephalopods, they swim by water propulsion. The giant squids, up to 18 meters in length, are the largest of today’s invertebrates. The octopus, with its eight equal-sized tentacles and its sack-like body, is adapted to life on the bottom, where it lives as a scavenger. The chambered nautilus is the last in a long line of shelled cephalopods, most of which have become extinct. The shell of the nautilus is coiled but not spiraled and is composed of a number of separate chambers. The animal lives in the largest chamber and uses the remainder as a sort of balloon to maintain its buoyancy in the water.

ACTIVITIES

e. Consider the Biocast model of the squid, as well as the preserved specimens. Using Figures 98d and e, identify the arms, tentacles, mantle and eyes. Which end goes first when this organism swims? If this animal has a shell, where is it located?

f. Provided is a Biocast model of a typical octopus. Compare this to the squid. How are they alike? How do they differ? Can you tell them apart?

g. Examine the shell of the chambered nautilus. Can you see the remains of the tiny canal connecting the chambers? Where did the adult live? How many times did it extend the shell?

Phylum Echinodermata

Some of the most familiar sea creatures, the starfish, are included in Phylum Echinodermata, along with several other interesting animals. They are of particular interest to us because studies of their embryology indicate a close kinship with the chordate phylum to which we belong. Most members of the phylum show the following characteristics:

A. They have radial symmetry and their construction is based on a plan of five, with major body parts typically occurring in fives or multiples of five. A bilateral larval form indicates non-radial ancestors, confirming a separate lineage from the other, more primitive, radically symmetrical invertebrates.

B. The skeleton is internal and made up of calcareous plates. These may articulate so as to give some flexibility to the organism, or they may be fused into a rigid shell. Hard spines may extend from these plates to give the organism a distinctive “spiny” appearance, hence the name Echinodermata (“spiny skin”).

C. Unique to this phylum is a sort of hydraulic apparatus, the water vascular system, which is used for locomotion by most echinoderms.

This interesting phylum has a long-standing and significant role in the history of the earth’s oceans. It is divided into five major taxonomic classes:

1. The crinoids or sea lilies are the most ancient and primitive of the echinoderms. They are best known from the fossil record, being widely distributed in the rocks, where their stem segments are locally known as “Indian money”. These animals may be envisioned as upside down starfish, anchored to the bottom by a long, jointed stalk. A few crinoid taxa still exist, but in nowhere near the number and diversity that they once enjoyed.

ACTIVITIES

h. Examine carefully the plaster cast of the crinoid “head”. Note the plates of which it is composed. See the arms. How many of them can you count?

2. The sea star or starfish is the most familiar animal in the phylum. These organisms consist of a central disk with a number of arms or rays arranged around it. At the center of the bottom or oral surface of the sea star is the mouth, with distinct ambulacral grooves extending to the tips of the arms. Within those grooves are two rows of tube feet. The upper or aboral surface is marked by little more than the sieve plate, a sort of filter connected to the water vascular system. The carnivorous starfish move on their tube feet and feed on mollusks and other small invertebrates.

ACTIVITIES

i. Select a preserved starfish, rinse it well and place it in a dissecting pan. DO NOT DISSECT THE ANIMAL. Using the labeled photographs in Figures 105a and b of the Photo Atlas, identify the following external parts: Central disk, arms, madreporite, mouth, ambulacral grooves and tube feet. Note the spiny skin and the fact that the sea star is somewhat flexible.

3. The brittle star has a more distinct central disk and arms, which are longer and more slender than the sea star. The water vascular system is poorly developed, with the few tube feet not functioning in locomotion. Members of this group are highly mobile, none-the-less, moving by a sort of “scrambling” motion of the arms. There are typically only five arms and the brittle stars are smoother, that is, less spiny, than most echinoderms. These animals, which act as predators or scavengers of the deep ocean bottom, are unusual in that they lack an intestine or an anus.

ACTIVITIES

j. Examine the plastomounts of the brittle star, note its characteristics and compare it to the sea star studied previously. What similarities exist? This organism is illustrated in Figure 106e. Know the common, phylum and kingdom names, which apply to this animal.

4. The sea urchins, sand dollars and sea biscuits have shells made of fused calcareous plates. The bodies of sea urchins do not have arms like the starfish and brittle stars; rather, they are more-or-less spherical, with movable spines projecting outward from the shell. Sand dollars are strongly flattened, and the sea biscuits are intermediate in thickness. While the urchins maintain the radial symmetry characteristic of the phylum, the other two taxa show signs of developing a secondary bilateral symmetry. The ambulacral areas of the sea urchin continue onto the aboral portion of the animal, dividing the shell into ten regions. The oral surface is flattened and bears a central mouth enclosing jaws with five teeth. On the sand dollars, the aboral ambulacral areas are shaped like the petals of a flower and radiate from the aboral center. Most of the organisms in this class are scavengers or detritus feeders. Some are capable of burrowing.

ACTIVITIES

k. While referring to Perry and Morton Figures 107a and b, examine the preserved sea urchins, as well as the sea urchin shell. See if you can find the following parts: Oral and aboral surfaces, spines, ambulacral areas, sieve plate, mouth and teeth.

l. Consider the specimens of sand dollar. Do you see evidence of a developing bilateral symmetry? Note the ambulacra on the aboral surface. Can you distinguish between these animals and the “fatter” sea biscuit? Both are shown in Figure 107c.

5. The sea cucumbers differ from the other echinoderms in that they have at least partially returned to a bilateral symmetry. These organisms lie on their sides, which puts the mouth on one end, as opposed to the oral-aboral orientation of the others. The sea cucumber is also more flexible, with the skeleton reduced to a series of small plates embedded in its leathery skin. The tentacles surrounding the mouth are used in feeding as the animal moves slowly over the sea floor on its tube feet, ingesting organic detritus.

ACTIVITIES

m. Examine the preserved specimen of the sea cucumber. Compare your specimen with those shown in Figures 107d and e of the Photo Atlas. Can you see the ambulacra on the animal’s body? Do those with functional tube feet differ from the others? Note the tentacles surrounding the mouth. Contrast the symmetry of this creature with that of the other echinoderms.