THE WORLD OF LIFE with Douglas Drenkow

Most mollusks are marine (some are attached to or buried in the sea floor), many are freshwater, and some gastropods are terrestrial (both day and night in moist environments but only at night in dry environments).

DESCRIPTION

"Hydrostatic" support (water-pressure) -- from the "hemocoel" (a blood-filled body cavity outside of the gut -- typically gives shape to the body. "Mantle" tissue (over the top and sides of the body) typically secretes a stony shell. A "mantle cavity" (a relatively large space between the mantle and the body) is typically present at one end of the body or the other (depending on the species). A muscular "foot" is typically present on the underside of the body.

In chitons, eight overlapping plates constitute the shell, covering the virtually headless body, flattened "dorso-ventrally" (from top-to-bottom) and about one inch to over a foot long.

In bivalves, a two-part, hinged shell houses the virtually headless body, flattened "laterally" (from side-to-side) and less than a centimeter to over a meter in size. The typically large, muscular, spade-like "foot" digs into the soft bottom of the aquatic environment (Atypically, oysters are cemented to rocks; and mussels secrete threads that anchor them in the rough tidal zone).

Shipworms and some other bivalves use their shells as drill bits.

Pearls are formed in many bivalves by their depositing shell material around particles of foreign matter that happen to get lodged in between their mantle and shell.

Gastropods either have a shell or have an evolutionary ancestor that had a shell (In other words, a slug is a shell-less snail; a snail is not a shelled slug). In gastropods, the single-piece shell is often cone-shaped or conveniently coiled-up above the broad, crawling foot (which may bear an "operculum", sealing shut the opening to the shell when the animal is withdrawn). The well-developed head typically bears tentacles and eyestalks. The body of a gastropod typically develops with "torsion" (described below -- basically, the tail-end grows around to the head-end). However, such gastropods as sea slugs (colorful and often decorated with elaborate outgrowths) develop with "detorsion" (the tail-end grows back to the tail-end). Gastropods range in size from less than inch to over a foot.

Amongst cephalopods, an external shell is present only in the nautilus (whose shell bears internal bulkheads and buoyant gas chambers): The shell of most cephalopods has been either reduced by evolution to an internal structure (the "cuttlebone" of cuttlefish) or entirely eliminated. Well-coordinated arms and tentacles grow from the head of a cephalopod and typically bear suction cups, used to grab onto prey or cling onto surfaces. Although some cephalopods are only an inch long, the giant squid -- the largest invertebrate on our planet -- is big enough to have warded-off attacks by sperm whales!

FEEDING HABITS

Many mollusks are scavenging or herbivorous; bivalves are filter-feeding; some marine gastropods are parasitic, on or in echinoderms or other mollusks; and some mollusks are carnivorous -- for example, "decollate" snails are used by gardeners to prey on pest snails that feed on plants; and cephalopods are formidable predators of many fish and invertebrates.

MOTION

Mollusks possess well-developed muscle tissues.

Such bivalves as scallops can scoot away from their predators underwater by quickly clapping their two valves together, like castanets.

Gastropods typically "glide" across surfaces (which they smooth with slimy secretions of mucous) by means of rippling undulations of their muscular foot, which may bear hair-like "cilia". In addition, the foot of such gastropods as abalones acts like a suction cup, protecting against wave action; and the foot of such gastropods as sea butterflies acts as a swim fin.

Although octopods usually crawl and some swim like jellyfish, most cephalopods -- with their tentacled head directed rearward and their streamlined, mantle-covered, often finned "viscera" (body mass) directed forward -- "fly" away from their predators underwater or chase down their aquatic prey by a more sophisticated form of "jet propulsion": The foot is modified as a siphon, used to concentrate and direct a blast of water delivered from the mantle cavity by the strong, muscular mantle wall.

DIGESTION

The gut of mollusks is "complete", with both a mouth and an anus. Except in bivalves, the mouth of a mollusk typically bears a "radula", a tongue-like organ that rasps food into particles, which it then conveys into the muscular "pharynx".

Typically in bivalves, "siphons" (modified, fused edges of the mantle that
grow upwards to great length in deeply burrowing species) pull in and push out a strong current of water; and a pair of typically large, folded, ciliated "ctenidia" gills -- enveloped by the fleshy mantle, within the shells -- functions not only in respiration but also in filter-feeding: The cilia of the ctenidia collect particles from the water current and carry them, in a string of mucous, to fingerlike "palps"; the palps then select food materials to be carried by their cilia to the nearby mouth.

The mouth of a cephalopod bears a pair of strong, parrot-like, poisoned jaws (forming a "beak"); and its rectal region (at the end of the digestive tract) bears a sac that can release an inky substance in the face of a predator, to visually -- sometimes narcotically -- confuse the enemy.

In most mollusks, a string of mucous, secreted by salivary glands in the mouth and carrying food particles, is pulled through an "esophagus" and wound-up on a rotating "style sac", within the stomach, where cilia sort waste particles -- directed into the intestine -- from food particles -- directed into a pair of "digestive glands".

The anus of mollusks typically opens into the mantle cavity, which is typically at the rear of the body. However, in most gastropods, the mantle cavity is near the head: During the development (and, presumably, the evolution) of gastropods, the body undergoes "torsion" -- the tail end growing around to the head end -- which turns the shell around and brings the mantle cavity forwards. This allows the head to be drawn-in more completely under the shell when the gastropod is threatened. In aquatic species, a water current flushes solid and liquid waste matter, discharged through the mantle cavity, away from the mouth; and in terrestrial species, the anus and the "ureter" (excreting urine) open outside of the mantle cavity.

RESPIRATION

Gases diffuse by osmosis, through selective cell membranes, across the body surface.

A pair of typically large, folded, ciliated "ctenidia" gills typically grows in the mantle cavity of aquatic mollusks, for gas exchange with the environment (oxygen taken in, carbon dioxide given off). In chitons, the
mantle cavity forms two troughs on either side of the body and bears several gills. In sea slugs and some other species, often elaborate and colorful outgrowths of the mantle may substitute for gills in gas exchange. A lung (a modified mantle) is present in many terrestrial gastropods, with air inhaled and exhaled through a small mantle cavity, opening towards the head (See "torsion", above).

CIRCULATION

Dissolved gases and other materials are typically carried throughout the body by an "open" circulatory system -- blood is pumped-out by the "ventricle" chamber of the muscular heart forward through the "aorta" blood vessel into the "hemocoel" (the collection of spaces surrounding the internal organs); and the blood returns through the hemocoel (not through any "veins") into the gills (where gases are exchanged) and, from them, to the pair of "auricle" chambers of the heart.

With a very active lifestyle, cephalopods have a more efficient, "closed" circulatory system (in which veins carry blood back from the bodily tissues), augmented by "secondary hearts", near the gills.

EXCRETION

Typically, a pair of "metanephridia" (primitive but open-ended, tubular kidneys) carry dissolved wastes from the hemocoel (the blood-filled body cavity), where they collect, to an opening in the mantle cavity, where the liquid is excreted, as urine.

COORDINATION

The growth, development, and activities of mollusks are under genetic and hormonal control.

"Nerve cords" and "ganglia" (nerve knots) process inputs and outputs. Most mollusks (except such completely shelled species as bivalves) are well "cephalized" (that is, they have a highly developed head), typically with sensitive tentacles and light-sensing eyes.

Cephalopods have eyes functionally much like our own -- able to distinguish shapes -- although they are probably near-sighted. Significantly, cephalopods are the most sensitive, coordinated, intelligent, and emotional of all invertebrates -- some can even rapidly change their often elaborate patterns of color with their "emotions", apparently forms of fear, aggression, love, etc. In addition, many deep-sea cephalopods are "bioluminescent" (able to chemically make their own light).

REPRODUCTION

Mollusks reproduce sexually. Although most mollusks have separate sexes, gastropods are typically "hermaphroditic" (each individual has the "gamete"-producing "gonads" of both sexes -- that is, the sperm-producing "testes" of the male and the egg-producing "ovaries" of the female). The gametes in some primitive species of mollusks exit through the kidneys, but in most species the gametes exit through a well-developed reproductive system.

Fertilization of the eggs by the sperms typically occurs in water (in the environment) or within the mantle cavity. "Copulation" (mating with physical contact) occurs in most gastropods, as well as in cephalopods -- the "genitals" (external sex organs) of both groups face forwards, thus producing more-or-less face-to-face contact during mating. With their arms intertwined, a specialized "copulatory arm" of the male cephalopod transfers a "spermatophore" (sperm packet) to the female. The fertilized eggs of mollusks are typically laid in the water (or in moist places, for terrestrial species). Cephalopods typically guard and clean their egg masses. Typically, a mollusk hatchling is a free-swimming "trochophore" larva, with a tuft of cilia at the "head" end, a ring of cilia around the middle, and the mouth and anus in the lower half. Many marine mollusks later develop into a "veliger" larva, which looks like a little adult and swims by means of a "velum" (a large, ciliated, typically funnel-shaped organ, sticking out from the shell). The young of cephalopods and terrestrial gastropods look like small adults, not dissimilar larvae; and as adults, land snails and slugs often remain dormant during periods of cold or dry weather.

Animals (Metazoa)

Doug@DouglasDrenkow.com

Photo of Cells: H.D.A. Lindquist, US EPA

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