The Molluscs

A Significant Space
Long ago in the Precambrian era, the most complex animals populating the seas were acoelomate. They must have been inefficient burrowers, and they were unable to exploit the rich subsurface ooze. Any that developed fluid-filled spaces within the body would have had a substantial selective advantage because these spaces could serve as a hydrostatic skeleton and improve burrowing efficiency.

The simplest, and probably the first, mode of achieving a fluid-filled space within the body was retention of the embryonic blastocoel, as in pseudocoelomates. This was not the best evolutionary solution because, for example, the organs lay loose in the body cavity.

Some descendants of Precambrian acoelomate organisms evolved a more elegant arrangement: a fluid-filled space within the mesoderm, the coelom. This meant that the space was lined with mesoderm and the organs were suspended by mesodermal membranes, the mesenteries. Not only could the coelom serve as an efficient hydrostatic skeleton, with circular and longitudinal body-wall muscles acting as antagonists, but a more stable arrangement of organs with less crowding resulted. Mesenteries provided an ideal location for networks of blood vessels, and the alimentary canal could become more muscular, more highly specialized, and more diversified without interfering with other organs.

Development of a coelom was a major step in the evolution of larger and more complex forms. All the major groups in sections to follow are coelomates.

Position in Animal Kingdom

1. Molluscs are one of the major groups of true coelomate animals.
2. They belong to the protostome branch, or schizocoelous coelomates, and have spiral cleavage and determinate (mosaic) development.
3. Many molluscs have a trochophore larva similar to trochophore larvae of marine annelids and other marine protostomes. Developmental evidence thus indicates that molluscs and annelids share a common ancestor.
4. Because molluscs are not metameric, they must have diverged from their common ancestor with annelids before the advent of metamerism.
5. All organ systems are present and well developed.

Biological Contributions

1. In molluscs gaseous exchange occurs not only through the body surface as in phyla discussed previously, but also in specialized respiratory organs in the form of gills or lungs.
2. Most classes have an open circulatory system with pumping heart, vessels, and blood sinuses. In most cephalopods the circulatory system is closed.
3. The efficiency of the respiratory and circulatory systems in the cephalopods has made greater body size possible. Invertebrates reach their largest size in some cephalopods.
4. They have a fleshy mantle that in most cases secretes a shell and is variously modified for a number of functions.
5. Features unique to the phylum are the radula and the muscular foot.
6. The highly developed direct eye of cephalopods is similar to the indirect eye of vertebrates but arises as a skin derivative in contrast to the brain eye of vertebrates.

The Molluscs
Mollusca (mol-lus'ka) (L. molluscus, soft) is one of the largest animal phyla after Arthropoda. There are nearly 50,000 living species and some 35,000 fossil species. The name Mollusca indicates one of their distinctive characteristics, a soft body. This very diverse group (Figure 16-1) includes chitons, tooth shells, snails, slugs, nudibranchs, sea butterflies, clams, mussels, oysters, squids, octopuses, and nautiluses. The group ranges from fairly simple organisms to some of the most complex of invertebrates, and in size from almost microscopic to the giant squid Architeuthis. These huge molluscs may grow to 18 m long, including their tentacles. They may weigh 450 kg (1,000 pounds). The shells of some giant clams, Tridacna gigas, which inhabit Indo-Pacific coral reefs, reach 1.5 m in length and weigh more than 225 kg. These are extremes, however, for probably 80% of all molluscs are less than 5 cm in maximum shell size. The phylum includes some of the most sluggish and some of the swiftest and most active invertebrates. It includes herbivorous grazers, predaceous carnivores, filter feeders, detritus feeders, and parasites.

Molluscs are found in a great range of habitats, from the tropics to polar seas, at altitudes exceeding 7000 m, in ponds, lakes, and streams, on mud flats, in pounding surf, and in open ocean from the surface to abyssal depths. Most of them live in the sea, and they represent a variety of lifestyles, including bottom feeders, burrowers, borers, and pelagic forms.

According to fossil evidence, molluscs originated in the sea, and most of them have remained there. Much of their evolution occurred along the shores, where food was abundant and habitats were varied. Only bivalves and gastropods moved into brackish and freshwater habitats. As filter feeders, bivalves were unable to leave aquatic surroundings. Only snails (gastropods) actually invaded the land. Terrestrial snails are limited in their range by their need for humidity, shelter, and presence of calcium in the soil.

Many kinds of molluscs are used as food. Pearl buttons are obtained from shells of bivalves. The Mississippi and Missouri river basins have furnished material for most of this industry in the United States; however, supplies are becoming so depleted that attempts are being made to propagate bivalves artificially. Pearls, both natural and cultured, are produced in the shells of clams and oysters, most of them in a marine oyster, Meleagrina, found around eastern Asia.

Some molluscs are destructive. Burrowing shipworms, which are bivalves of several species (see Figure 16-27), do great damage to wooden ships and wharves. To prevent the ravages of shipworms, wharves must be either creosoted or built of concrete (unfortunately, some ignore the creosote, and some bivalves bore into concrete). Snails and slugs frequently damage garden and other vegetation. In addition, snails often serve as intermediate hosts for serious parasites. The boring snail Urosalpinx rivals sea stars in destroying oysters.

In this section we explore the various major groups of molluscs, including those that apparently met with little evolutionary success (classes Caudofoveata, Solenogastres, Monoplacophora, and Scaphopoda). Members of class Polyplacophora (chitons) are common to abundant marine animals, especially in the intertidal zone. Bivalves (class Bivalvia) have evolved many species, both marine and freshwater. Largest and most intelligent of all invertebrates are in the class Cephalopoda (squids, octopuses, and others). Most abundant and widespread of molluscs, however, are snails and their relatives (class Gastropoda). Although enormously diverse, molluscs have in common a basic body plan, which is described later in the section. It seems peculiar, though, that molluscs have failed to exploit the coelom. The coelom in molluscs is limited to a space around the heart, and perhaps around the gonads and part of the kidneys. Although it develops embryonically in a manner similar to the coelom of annelids, the functional consequences of the space are quite different. Some zoologists believe that molluscs arose from a flatworm-type ancestor separately from annelids and that their coeloms are not homologous.

Characteristics of Phylum Mollusca

1. Body bilaterally symmetrical (bilateral asymmetry in some); unsegmented; often with definite head
2. Ventral body wall specialized as a muscular foot, variously modified but used chiefly for locomotion
3. Dorsal body wall forms pair of folds called the mantle, which encloses the mantle cavity, is modified into gills or lungs, and secretes the shell (shell absent in some)
4. Surface epithelium usually ciliated and bearing mucous glands and sensory nerve endings
5. Coelom limited mainly to area around heart, and perhaps lumen of gonads and part of kidneys
6. Complex digestive system; rasping organ (radula) usually present; anus usually emptying into mantle cavity
7. Open circulatory system (mostly closed in cephalopods) of heart (usually three chambered), blood vessels, and sinuses; respiratory pigments in blood
8. Gaseous exchange by gills, lungs, mantle, or body surface
9. One or two kidneys (metanephridia) opening into the pericardial cavity and usually emptying into the mantle cavity
10. Nervous system of paired cerebral, pleural, pedal, and visceral ganglia, with nerve cords and subepidermal plexus; ganglia centralized in nerve ring in gastropods and cephalopods
11. Sensory organs of touch, smell, taste, equilibrium, and vision (in some); eyes highly developed in cephalopods
12. Internal and external ciliary tracts often of great functional importance
13. Both monoecious and dioecious forms; spiral cleaveage; larva primitively a trochophore, many with a veliger larva, some with direct development.