Read an Excerpt
Seashells of North America
By R. Rucker Abbott, George F. Sandström
St. Martin's PressCopyright © 1996 Golden Books Publishing Company, Inc.
All rights reserved.
WHAT IS A MOLLUSK?
Over a million species of invertebrate animals have been classified into ten or more major groups, or phyla. One of the oldest and most successful groups, with about 60,000 living species, is the phylum Mollusca. The mollusks include such familiar forms as clams, oysters, snails, and conchs. Although they may be superficially classified as shelled animals, the mollusks as a group are so diverse that a simple definition is difficult. Mollusks were originally marine animals. Present-day species have become adapted to life in a great number of different ecological niches. They live in mud, sand, or rocks and on land as well as in fresh and salt waters. A few are parasitic. Other invertebrates, such as crabs, produce shells of a different origin.
The mollusk body is divided into three regions: the head, the foot, and the visceral mass. The head is well-developed in the snails and squids, which have eyes, tentacles, and a well-formed mouth. In the clams, or bivalves, the head does not appear as a distinct structure. Associated with the body, though sometimes classified as a separate basic structure in the mollusks, is a thin sheet of tissue — the mantle — which secretes the limy shell characteristic of most mollusks. The mantle cavity between the mantle and the visceral mass contains the gills and the waste ducts, from intestines and the kidneys.
Inside the mouth cavity, mollusks usually have a ribbonlike set of teeth known as the radula. These rasping teeth vary in number and size. The radula is absent in the pelecypods, or bivalves.
The visceral mass contains the internal organs of circulation, respiration, excretion, digestion, and reproduction. The heart has two chambers and circulates colorless or blue blood. The paired kidneys, with tubes, open into the mantle cavity. The digestive tube begins at the mouth, continues through a stomach, which often has dead-end sacs, then through a long intestine that also terminates in the mantle cavity. In mollusks the sexes are usually separate, but there are many hermaphroditic species with both male and female sex organs. In most mollusks the fertilized egg develops into a trochophore larval stage that is free-swimming and bears bands of minute cilia. This is followed by a more advanced larval stage, the veliger in most groups (see here)
MAJOR GROUPS OF MOLLUSKS
The phylum Mollusca is divided into six classes on the basis of major differences in such anatomical features as the foot, shell, mouth parts, and breathing organs. The various classes represent widely differing degrees of evolution and specialization, as well as numbers of species. There are representatives of all the classes in the oceans, but only two have invaded fresh waters, namely the bivalves and univalves. The latter are also found on land.
Class MONOPLACOPHORA, containing the primitive gastroverms, is represented by only six living deep-sea species, although many fossil species are known. The shells are spoon-shaped, resembling limpets, but on the inside there are five or six pairs of muscle scars. The internal organs and gills are paired and each arranged in a separate segment. There are no eyes or tentacles. A row of radular teeth is in the pharynx. The anus is at the posterior end.
Class AMPHINEURA, or chitons, are rock-dwelling marine mollusks having eight shelly plates that arise embryologically from a single shell gland. The rather primitive animal is encircled by a leathery border, the girdle, which holds the shelly plates in place. Underneath is a large, broad foot and a head that lacks eyes and tentacles. There is a well-developed radula. Sexes are separate. Some species brood their young. Most of the 600 or so species are herbivorous, but some are carnivorous.
Class GASTROPODA is the largest class of mollusks, with about 40,000 living species. Univalves usually have a single, coiled or cap-shaped shell. A few lack shells. Most have a radula, well-developed tentacles, and a pair of eyes. Many produce an operculum on the back of the foot; this serves to seal the opening of the shell. In all gastropods, the late embryonic stage undergoes a peculiar twisting of 180°, called "torsion," so that many of the posterior internal organs face forward into the mantle cavity.
The univalves are commonly divided into three subclasses. The Prosobranchia snails are the most numerous and are mainly aquatic. They have gills in the mantle cavity. There are two orders: the herbivorous, marine Archeogastropoda, such as the top and turban snails, which have numerous radular teeth, and the omnivorous fresh- and salt-water Caenogastropoda, such as the conchs and mud snails, which have relatively few radular teeth. Snails of the subclass Opisthobranchia are marine snails, such as the sea hares and bubble snails, which commonly have a much-reduced shell. The gills are generally posterior to the heart. Male and female reproductive organs are found in each individual. Most are herbivorous, but many are parasitic on clams and other sea creatures. The subclass Pulmonata, not treated in this book, includes the land snails, garden slugs, and pond snails, all of which have a modified "lung" and usually lack an operculum. All are hermaphroditic and feed mainly on plants.
The class BIVALVIA, containing the bivalves or pelecypods, is the second largest group of mollusks and has about 10,000 living species. Most are marine, but many live in fresh water. The soft parts are enclosed within two hinged shells, or valves, hence the term "bivalve" for this group. The valves are pulled together by one or two large adductor muscles and are kept ajar by a chitinous, elastic ligament. Interlocking teeth in the hinge of each valve add strength to the connection and prevent the valves from slipping. The foot is well-developed in clams that burrow, but it is small in attached forms, such as the oysters. Some groups, like the mussels and pen shells, spin a byssus, a clump of chitinous threads that serves as an anchor. Bivalves do not have a head or radula. Eyes sometimes occur along the edge of the mantle, as in the scallops and thorny oysters.
The posterior section of the mantle is often modified into two tubular extensions, the siphons, one of which brings water and algal food into the mantle cavity, and the other of which expels wastes. The siphons are long in species that burrow deeply into sand, rock, or wood. Food particles stick to mucus that is transported by many hairlike cilia on the gills to a groove near the mouth. A pair of fleshy pads, the palpi, then push the food into the mouth.
In most bivalves the sexes are in separate individuals, although scallops and others may be hermaphroditic. In some oysters, a female phase is later replaced by a male phase. Eggs and sperm of bivalves are shed into the open sea water. The developing young are free-swimming for many days or weeks before settling to the bottom. In fresh-water mussels, the larvae, called glochidia, attach themselves to the gills of fish and suck the blood of the host until they are ready to drop to the bottom as young mussels.
Most bivalves move freely through the sand or mud substrate, but many, such as the oysters and jingles, are attached permanently to rocks or wood. Many bore into wood and rocks.
Both the gills and shell hinges have been used in the higher classification of the bivalves, but no system has been universally accepted. There are four main types of gills, as illustrated on the opposite page. The protobranch gills, found in the order Protobranchia, are flat and platelike. They are regarded as the most primitive. The filibranch gills, found in arks, mussels, oysters, and scallops (order Filibranchia), are long curtains folded back against themselves. The eulamellibranch gills, in cockles and clams (order Eulamellibranchia), are similar to filibranch gills except that the curtains are united by cross-channels. The septibranch gills, in dipper clams (order Septibranchia), are mere slits in the thin wall that closes off the respiratory chambers. The Palaeoconcha, containing the awning clams, have filibranch-like gills, but this order is considered unique because of its toothless hinge, lack of siphons, and flat-ended foot.
The class SCAPHOPODA contains the tusk, or tooth, shells. These mollusks live in marine waters buried in mud, with the narrow posterior end of their shell protruding into the water. The shell, from 0.5 to 5 inches long, is tubular and open at both ends. A conical foot projects from the larger posterior end. Numerous, small, tentacle-like filaments, the captacula, capture foraminifera and other small organisms and pass them to the mouth, within which is a radula. No true head, eyes, or gills are present. Water passes in and out of the posterior end, with respiration taking place through the walls of the mantle. There is no heart or pericardium. The blood circulates through contracting sinuses. The sexes are separate, and the sperm or eggs are passed into the sea from two small kidney openings. Most of the 300 living species live at depths of from 10 to 100 feet, but some exist at a depth of 3 miles.
The class CEPHALOPODA, which is entirely marine, includes the most active of the mollusks and, indeed, of all the invertebrates. One species of squid is the largest known living invertebrate. The squids, octopuses, and nautiluses have a cartilaginous brain case and a well-developed nervous system that permits great speed, strength, and alertness. The large eyes reach a perfection found nowhere else in the invertebrates. A series of prehensile tentacles surround the mouth, which is armed with a parrotlike, chitinous beak and, within the pharynx, with a strong set of radulae. Most of the 600 species are predatory carnivores. The sexes are separate, and the heavily yolked eggs are laid in grapelike festoons of jelly material. There is no trochophore nor is there a veliger larval stage, as there is in other mollusks. Many forms have the ability to squirt out clouds of brown or purple ink, evidently as a defense mechanism. Some species display vivid and rapid changes of color by the muscular control of chromatophore pigment cells. Propulsion is accomplished by squirting water from a funnel-shaped fold of the mantle edge. Deep sea species bear luminescent organs of various colors, and the ink of some may give off a slight glow. In most species, when mating takes place, the male transfers bundles of sperm to the female by means of a modified arm called the hectocotylus.
The shells of the cephalopods are very diverse. In the subclass Nautiloidea, represented today by only one family in the Pacific, the shell is large, external, and divided into numerous gas-filled chambers. In the subclass Coleoidea, which includes the squids and octopuses, the shell may consist of an internal, chalky cuttlefish "bone," or a thin, flat, rodlike support, as is found in squids. The octopus does not have a shell. The white, parchment-like "shell" of the paper nautilus, or Argonauta, is actually an egg case secreted by two flat arms of the female. The Spirula deep-sea squid has a chambered, coiled shell resembling a miniature ram's horn.
The origin and the early history of the mollusks are shrouded in mystery because not many fossil remains are known from the Pre-Cambrian period, when this phylum probably came into being, more than 600 million years ago. By examining the numerous fossils of the Cambrian and later periods, and by making comparative studies of the anatomy and embryology of living forms, reasonable deductions may be drawn. The immediate ancestors of the first mollusks were probably simple, shell-less animals having segments like those still found in today's annelid worms and arthropods. The development of the mantle and the production of a protective calcareous shell set the stage for the evolution of the greatly diverse classes of mollusks.
The univalves and bivalves appeared in the Early Cambrian, about 600 million years ago. Evidently, it took some of the bivalves another 200 million years to enter fresh water, and it took certain univalves at least 300 million years to evolve into land and fresh-water forms. The chitons and tusk shells appeared fairly soon after the univalves, but the production of new forms and attempts to try new habitats were largely unsuccessful. The gastroverms, represented today by only five known species of Neopilina, are the closest known forms to the ancestral mollusks, but from the Cambrian to Recent times they have varied little and given forth few species.
The cephalopods have a long, well-documented, and interesting history. The now extinct ammonoid subclass of huge, coiled shells first appeared in the Lower Devonian, some 400 million years ago. They flourished in great numbers until the Cretaceous, when suddenly and mysteriously they all vanished. The nautiloid subclass first came on the scene as early as the Upper Cambrian and abounded in the seas for about 200 million years. Gradually they waned in number and today they are represented only by four or five living species of Chambered Nautilus in the southwest Pacific Ocean. Most of today's cephalopods, such as the squids and octopuses, belong to the subclass of two-gilled Coleoidea. Perhaps because many were shell-less, their remains in the fossil records do not appear until the late Carboniferous, some 265 million years ago.
Most of today's genera of marine univalves and bivalves blossomed into dominance about 65 million years ago when great geologic and biologic changes heralded the end of the Cretaceous period. With the advent of the Tertiary period, the ammonoids died out, many primitive families of univalves disappeared, and such genera as Conus, Cassis, Strombus, Venus, and Ensis came into being. Some groups, like the cones, exploded into hundreds of species, while others, like the Strombus conchs, produced but few new kinds. In the last million years the land and fresh-water mollusks have evolved very rapidly, but the marine forms seem to be decreasing in the number of species.
The manner of sexual reproduction and larval development varies considerably among the various classes and families of mollusks. In all chitons and tusk shells and in most cephalopods and bivalves the animals have separate sexes. In some bivalves, such as the scallops and cockles, and in many gastropods, including all of the pulmonate snails, the individuals are hermaphroditic, each with functional male and female organs. In some fresh-water snails, the eggs may develop parthenogenetically without being fertilized by sperm.
Sperm or eggs produced in the gonads pass down a separate duct, which exits either into the kidney tube or directly into the mantle cavity. The eggs may be fertilized in the open sea water or within the oviduct of the female. In some families of gastropods there are two types of sperm — the very small, numerous normal sperms which fertilize the eggs and the large spermatozeugma, which is a swimming, undulating carrier of the small sperm. The latter type is found in such genera as Epitonium, Janthina, and Cerithiopsis, which lack a penis and require the carrier-sperm to gain entrance into the oviduct.
In the cephalopods, a torpedo-shaped tube of chitin containing a dense mass of sperm is plucked by the male from his own genital opening by a specially modified arm, known as the hectocotylus. These spermatophores are then placed inside the mantle cavity of the female, where they eventually work their way into the oviduct to fertilize the eggs.
Some snails, particularly the land forms, practice a simple form of courtship in which they entwine themselves before pairing. A calcareous shaft, or "love dart," is exchanged by the partners in order to stimulate sexual behavior. In some snails the penis bears numerous hooks. In the hermaphroditic opisthobranchs, such as Bulla and Hydatina, chains or rings of mating individuals may be found at certain seasons.
In all mollusks except the cephalopods the fertilized egg and the subsequent cells formed from it divide completely in half. However, the cleavage is oblique, not parallel to the polar axis of the egg. This results in a spiral arrangement of the dividing cells, a form common to flatworms and mollusks. In the cephalopods, the cleavage does not divide the egg completely; rather, it forms a cap of divided cells at the top third. The remainder of the egg becomes the yolk, which later develops into a yolk sac. Under ideal conditions, the first cleavage of the egg takes place within three or four hours after the sperm enters.
The size of the egg differs from one species to another, even within the same genus. Larger eggs contain a greater amount of nutritive yolk and usually complete their development within the egg capsule. Those species with smaller eggs generally hatch sooner and give rise to a free-swimming larval stage.
Excerpted from Seashells of North America by R. Rucker Abbott, George F. Sandström. Copyright © 1996 Golden Books Publishing Company, Inc.. Excerpted by permission of St. Martin's Press, a division of Random House, Inc.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.
Table of Contents
ContentsHow to Use This Book,
What Is a Mollusk?,
Major Groups of Mollusks,
Shell Structure and Pigmentation,
Nervous System and Sense Organs,
Commensalism and Parasitism,
Dispersal and Abundance,
Marine Faunal Provinces,
Guide to Collecting Areas,
Cleaning and Preserving,
The Shell Collection,
Studying and Observing Mollusks,
Marine Aquariums for Mollusks,
Nomenclature and Classification,
CLASS GASTROPODA (Univalves),
CLASS AMPHINEURA (Chitons),
CLASS SCAPHOPODA (Tusk Shells),
CLASS BIVALVIA (Bivalves or Pelecypods),
CLASS CEPHALOPODA (Octopus, Squids),