Download The Life of a Mollusk The mollusks constitute one of the largest

The Life of a Mollusk
The mollusks constitute one of the largest phyla of animals, both in numbers of
living species (at least 93,000, and perhaps many more) and in numbers of
individuals.
All mollusks have a soft body (their name is derived from the Latin word
mollus, meaning "soft"), which is generally protected by a hard, calciumcontaining shell. In some forms however, the shell has been lost in the course of
evolution, as in slugs and octopuses, or greatly reduced in size and internalized, as
in squids.
Structurally, mollusks are quite distinct from all other animals. However, all
modern mollusks have the same fundamental body plan. There are three distinct
body zones: a head-foot, which contains both the sensory and motor organs; a
visceral mass, which contains the well-developed organs of digestion, excretion, and
reproduction; and a mantle, a specialized tissue formed from folds of the dorsal body
wall, that hangs over and enfolds the visceral mass and that secretes the shell. The
mantle cavity, a space between the mantle and the visceral mass, houses the gills;
the digestive, excretory, and reproductive systems discharge into it.
Mollusks are also characterized by a toothed tongue, the radula, composed
primarily of chitin. The radula serves both to scrape off algae and other food
materials and also to convey them backward to the digestive tract. In some species,
it is also used in combat.
Digestion and Excretion
The digestive tract is complete and ciliated, with a mouth, anus and complex
stomach. The pattern of the stomach varies according to the mollusks diet. Food is
taken up by cells lining the digestive glands arising from the stomach, and then is
passed into the blood. Undigested materials are compressed and packaged, then
discharged through the anus into the mantle cavity and are carried away from the
animals in the water currents. This packaging of wastes in solid form prevents
fouling of the water passing over the gills.
Nervous System and Sensory Capability
Mollusks have a relatively complex nervous system, which varies from species to
species reaching the height of complexity at the octopus. The octopus is thought to
be among the most intelligent of all invertebrates, with a mental capacity likened to
that of a domestic cat. Sensory ability in some mollusks (notably the cephalopods)
is considerable, with a variety of organ systems, as well as large, complex eyes. The
eyes of the giant squid are the largest in the animal kingdom, approaching the size
of dinner plates. It has recently been demonstrated that squid can successfully
locate and capture transparent prey in the water by means of a specialized
polarization vision.
Respiration and Circulation
Excluding cephalopods, mollusks have an open circulatory system, meaning the
blood does not circulate entirely within vessels but is collected from the gills,
pumped through the heart, and released directly into spaces in the tissues from
which it returns to the gills and then to the heart. Cephalopods, whose vigorous
activities require that the cells be supplied with large quantities of oxygen and food
molecules, have a closed circulatory system of continuous vessels and accessory
hearts that propel blood into the gills. The presence of discrete respiratory and
circulatory systems has led to an improved capacity for oxygen uptake and
distribution, and hence an increase in body mass. Mollusks include the largest and
most advanced of living invertebrates with the cephalopods, including the
monstrous Giant Squid, which can reach a total length, including tentacles, of 18
meters.
Locomotion
Locomotion amongst the mollusks varies considerably, and is dictated by the
structure of the foot. Herbivorous forms are commonly gliders, moving on waves of
muscular contraction. However, many carnivorous forms have achieved more
advanced forms of locomotion. Cephalopods swim actively by a type of jet
propulsion, in which water is rapidly expelled from the mantle cavity via the
siphon. In the streamlined squid this technique has enabled to animals to achieve
the fastest speeds of any aquatic invertebrate. The Cuttlefish and the Sea Hares
rely upon undulating lateral fins for highly maneuverable locomotion. In the
bivalves the foot has developed into a tool for burrowing, which can be remarkably
rapid for example in the common Razor Shells.
Mollusks exhibit a tremendous diversity of form and behavior. The three major
classes range from largely sedentary or sessile suspension-feeding animals, such as
clams and oyster (class Bivalvia), through aquatic and terrestrial snails and slugs
(class Gastropoda), to the predatory cuttlefish, squids, and octopuses (class
Cephalopoda).
Class Bivalvia
The approximately 7,500 living species of bivalves include such common animals as
clams, oysters, scallops, and mussels. They derive their name from the two parts,
or valves, into which the shell is divided. One or two large, well-developed adductor
muscles are used to close the shell swiftly and tightly in times of danger. These
muscles are so strong in scallops that the animals can move swiftly through the
water by clapping their shells together, thereby eluding predators such as starfish.
Most bivalves are dioecious (have separate sexes; thus, not hermaphroditic) and
fertilization can be either internal or external, depending on the species.
Freshwater clams differ in that their fertilization is internal, with brooding taking
place in the gill chamber. Specialized "brood pouches", in which the young are
protected during their early development, are found in hermaphroditic species as
well as in the females of some species with separate sexes.
The bivalves have sensory cells for discrimination of touch, chemical changes, and
light. The scallop has quite complex eyes; a single individual may have a hundred
or more eyes located among the tentacles on the fringe of the mantle. The lens of
this eye cannot focus images, however, so it does not appear to serve for more than
the detection of light and dark, including passing shadows cast by other moving
organisms.
Abundant in both salt and fresh water, most adult bivalves are sedentary,
herbivorous filter feeders, using currents set up by cilia on their gills to bring in
food particles, usually microscopic algae. A few bivalves, such as scallops, are able
to move rapidly by clapping their valves together. Others are able to move slowly by
muscular action of their foot. Still others are completely sessile.
Class Gastropoda
The gastropods, which include the snails, whelks, periwinkles, abalone, and slugs,
are the largest group of mollusks (at least 37,500 living species). They have either a
single shell or no shell. Gastropods are common in both salt and fresh water and on
land. Class Gastropoda includes herbivores, omnivores, a wide variety of
specialized carnivores, scavengers, and even some parasites.
Gastropods, which lead a more mobile, active existence than bivalves, have a
nervous system with as many as six pairs of ganglia connected by nerve cords.
There is a concentration of nerve cells in the tentacles, at the anterior end. In some
of the animals, the eyes are quite highly developed.
In some gastropods, the primitive form of reproduction--separate sexes with
external fertilization--is retained. In most gastropods, however, fertilization is
internal, and hermaphroditism has evolved repeatedly in this group. The
simultaneous hermaphroditism found in many snails and slugs probably resulted
from the difficulties of such slow-moving animals in finding mates. In some species,
the animals are males when they are younger, then females when they are older
and larger.
Class Cephalopoda
The cephalopods (the “head-foots”) are, in many respects, the most evolutionarily
advanced animals to be found among the invertebrates. The 600 living species in
this strictly marine class rival the vertebrates in complexity and, in some cases, in
intelligence. Active predators, they compete quite successfully with fish.
Although obviously mollusks, the cephalopods have become greatly modified.
The large head has conspicuous eyes and a central mouth surrounded by arms,
some 90 in the chambered Nautilus, ten in the squid, and eight in the octopus. The
octopus body seldom reaches more than 30 centimeters in diameter, but giant
squids sometimes attain sea-monster proportions.
Nautilus, as the only modern shelled cephalopod, offers an indication of some of
the steps by which this class disposed of the shell entirely. The animal occupies
only the outermost portion of its elaborate and beautiful shell, the rest of which
serves as a flotation chamber. In the squid and its relative, the cuttlefish, the shell
has become an internal stiffening support, and in the octopus, it is lacking entirely.
Freedom from the external shell has given the mantle (the outermost layer or
body wall, or a soft extension of it) more flexibility. The most obvious effect of this
is the jet propulsion by which cephalopods dart through the water. Usually, water
taken into the mantle cavity bathes the gills and is then expelled slowly through a
tube-shaped structure, the siphon; but when the cephalopod is hunting or being
hunted, it can contract the mantle cavity forcibly and suddenly, thereby squirting
out a sudden jet of water. Contraction of the mantle-cavity muscles usually shoots
the animal backward, head last, but the squid and the octopus can turn the siphon
in almost any direction they choose. Cephalopods also have sacs from which they
can release a dark fluid that forms a cloud, concealing their retreat and confusing
their enemies. These colored fluids were at one time a chief source of commercial
inks. Sepia is the name of the genus of cuttlefish from which a brown ink used to be
obtained.
The cephalopods have well-developed brains, composed of many groups of
ganglia, in keeping with their highly developed sensory systems and their lively,
predatory behavior. These large brains are covered with cartilaginous cases. The
rapid responses of the cephalopods are made possible by a bundle of giant nerve
fibers that control the muscles of the mantle. Many of the studies on conduction of
nerve impulses are made with the giant axons of squids, which are large enough to
permit the insertion of electrodes.
In cephalopods, the sexes are always separate, and fertilization is internal.
Courtship and mating behavior are complex, and males often fight for access to
females. Fertilization occurs when the male uses one of his arms to transfer the
packets of sperm, called spermatophores, from his mantle cavity to the mantle
cavity of the female. The female produces an intricate, gelatinous egg mass in
which the developing embryos are protected until they hatch as miniature adults.
In octopuses and some other cephalopod genera, the mother guards the egg masses,
cleaning and aerating them.