Mollusca (soft body)
Snails, clams, chi tons, octopuses, squids, and tusk shells, among others, belong to the phylum Mollusca. The soft body is un segmented, but may be divided into an anterior head (very reduced in dams), dorsal mantle, and ventral foot. At the head or front end of the animal is a mouth; at the opposite end is an anus. The foot in clams, snails, chitons, and tusk shells allows for movement to take place by creeping. This foot is divided into a
number of arms in squids and octopuses. The soft tissue of the mantle lines and secretes the calcareous shell.
Inside the body are such vital organs as a heart and vascular system, nervous system, liver, digestive and excretory organs, and an alimentary tract. Respiration is carried on by gills located under the mantle and the shell. Most land snails are hermaphroditic (male and female reproductive functions carried on by one animal), as are some marine mollusks. The others reproduce as sexually separate parents. Because of the great variations in shell morphology, the nature of the shell will be discussed under each class.
Mollusks first appeared in the Cambrian (543-489 million years ago), and the phylum includes many modern species. They show a wide range in adaptation to environment and are found from 34,000 foot-deep ocean trenches to inter-tidal marine environments. They occur in fresh water and in many land environments to 18,000 feet above sea level. Many mollusks are excellent aids in relative time correlation of rocks.
- Bivalvia (class) - Bivalves (or "two shells") include clams, oysters, and mussels. Unlike the other mollusks, pelecypods (another name for bivalves, the word means "hatchet foot") have a very reduced head. The mouth is located at the front of the body. The animal moves by extending its ventral foot and drawing itself forward. Other bivalves, such as oysters and mussels, are fixed in one place. The shell is closed by muscles that extend between the shells. An elastic hinge that connects the two shells works like a spring to open the shells. The hinge is up (dorsal), and the valves enclose the right and left sides of the animal. The beak, or highest point, of the shell is located off the center of the shell as viewed from the side. Bivalve shells are usually the same size, but are mirror images (left and right) of each other. Brachiopod valves differ in size and shape, and are not mirror images. The concentric lines that radiate around the beak of the bivalve shell are growth lines. Bivalves appeared In the Early Cambrian (ca. 517 million
years ago), and the oldest known bivalve in earth was first described from Troy, New
- Gastropoda (class) - The gastropods (or "stomach foot") include land and water snails
(with shells) and many forms (as garden slugs) without shells. The gastropod body generally resembles the clam's, except that the gastropod has a well-developed head and a broad foot used in creeping. This head has a pair of eyes on tentacles, and a mouth with a rasping tongue for shredding food. Some marine snails use their tongue to cut a cylindrical hole through clam shells, after which the clam's body is cut into small pieces and eaten. These borings are common in modem and fossil clam shells. The snail shell is usually coiled, tightly or loosely, and consists of only one piece. Each complete coil is called a whorl. As the animal grows, the whorls coil in a helix down from the apex. The shell shows growth lines, and frequently nodes, grooves, and ridges. The gastropods exceed all the other classes of mollusks in variety of species. They appeared during the Early Cambrian (ca. 530 million years ago) and are abundant at present.
- Cephalopoda (class) - The cephalopods (or "head foot") are represented by modern octopuses, squids, and the pearly nautilus. All cephalopods live in sea water. Unlike the
slow-moving snail, many cephalopods are strong, fast swimmers. They take water into
their mantle cavity, and squeeze it out to swim by jet propulsion. Some ancient cephalopods, had straight shells. Other cephalopods have a reduced or internal shell; the present-day octopus has none at all. Straight cephalopod shells are cone-shaped. The body was in the largest part of the shell and projected from the opening. As the animal grew, it moved forward, building the shell larger as it did. The space that it left behind its body was partitioned off by transverse walls called septa. A shell may contain scores of these walls. In the modern Nautilus, one septum is formed each month. Thus, the number of septa in a modern or fossil shell gives the age of the animal when it died. Nitrogen gas fills the spaces between the septa and serves to buoy up the shell and the body. A straight tube called a siphuncle runs through the septa and served to pump water out and replace it with nitrogen. The junction of the septa to the shell wall is often preserved in internal molds as
simple or complex seams. The earlier cephalopods were straight shelled, which was awkward in swimming. Because of this, coiled cephalopods arose early in the Paleozoic and became more abundant than straight cephalopods.
- Cephalopoda (class) - The coiled cephalopods should not be confused with gastropods. The coiled cephalopod shell is usually coiled in one plane; the gastropod shell is wound into a spiral. The internal shell structure of the coiled cephalopod is basically the same as in the straight cephalopod. The advantages of a coiled shell are that the center of gravity is close to the body. This allows for greater control of direction during swimming, and the rounded shape is stream-lined. Coiled cephalopods (as the modern pearly nautilus) had a mother-of-pearl lining of the shell.
- Tentaculites (genus) - Tentaculites is an extinct marine animal that is known from fossil shells. These are small, cone-shaped, protective coverings characterized by ridged rings. Septa were secreted behind the animal as it grew larger and forward in its shell. The tip of some specimens may have a small bulb that is the larval shell. Tentaculites lived from the Silurian (440-418 million years ago) to the Devonian (395 million years ago). Predation by advanced cephalopods and fish may have contributed to its extinction. Tentaculites may have been a mollusk, but the fine-grained calcite crystals that compose the shell a re unknown in other mollusks.
- Hyolithes (genus) - Hyolithes is representative of an extinct group of marine animals called hyoliths that appeared in the Early Cambrian (ca. 530 million years ago) and went extinct at the end of the Permian (245 million years ago). With a heavy conical shell and an operculum (a "trap door "), hyoliths probably rested on the sea floor. Fossilized mud-filled intestines have been foun d in the shells; this suggests they were sediment-feeders. Hyoliths may have been mollusks, but their fine-grained shells are dissimilar to those of known mollusks.
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