Introduction

     Ever since I was a toddler visiting my grandparents in Cape Breton Island, Nova Scotia, I was intrigued and fascinated by seashells and the creatures that build and inhabit them. I soon began a shell collection and started to read up on these magnificent organisms. I discovered at this early age that I had profound interest and curiosity for mollusks.

     I am neither the first nore shall I be the last to share this fascination. Shells, and the creatures they contain, have intrigued humans since the dawn of civilization. The study of shells is an ancient one, dating back to Antiquity. Such great scientists as Aristotle and Pliny have written many words upon the subject. In nineteenth-century Europe, it was known as "the Queen of natural history studies" while in Japan it was dubbed "the emperor?s science."

General Anatomy

     The word mollusk is derived from the Latin word mollis meaning soft and referring to the soft-bodied creatures. Mollusks are, as stated, soft-bodied organisms comprising of a visceral mass, a muscular foot and in most mollusks a head. The head, in addition to bearing a mouth, often bears sensory tentacles and eyes. The mouth usually leads into a chamber known as the pharynx, which usually contains a radula, a flexible, ribonlike structure consisting of numerous rows of minute teeth, used in obtainig food.

     A mollusk's digestive system is composed of three main sections. The anteriormost section consists of the mouth and esophagus. The specialized radula in the mouth is used for feeding in the majority of mollusks and is found in this phylum only. Pelecypods, however, are filter feeders and therefore lack the characteristic radula. In its place, they have siphons to intake and expel water. The mid-section of the digestive tract consists of the stomach and liver. In many mollusks a crystalline style, flexible rod composed of mucus and protein, situated in the mid-gut, is essential for secreting digestive enzymes and serves as a "strirring rod" for stomach contents. The last section of the digestive tract consists of the intestine and anus. The mollusk?s digestive system is covered with microscopic hair-like projections and contains several sub-divisions for specialized digestive functions.

     A mollusk's nervous system is generally formed of a system of ganglionic centers, connecting nerve cords, sensory organs and motor organs. Nerves from the cerebral ganglia usually innervate the head region and in higher forms (snails, octopus and squid) there is a tendency to concentrate ganglia in the head region (cephalization). Nerves from the pallial and pedal ganglia generally innervate the foot, mantle and body wall, while the viscera are often innervated by stomatogastric ganglia. In lower mollusks the system is ladderlike, with ganglia distributed in bilateral pairs connected by long, longitudinal nerve cords and shorter, laterally running nerve cords.

     Mollusks breathe by way of gills, except in the case of land mollusks which have evolved a modified lung in a cavity associated with the mantle. Most mollusks have an open circulatory system, with three-chambered hearts. Blood is oxyginated by the gills or modified lung and then passes to the heart, which pumps the blood into sinuses where it bathes body tissues directly before returning to the gills. Exceptions include the cephalopods, which have closed circulatory systems.

     Mollusks usually have their own sexe but some cases of hermaphroditism have been found. Courtship is either nonexistent, as in the more primitive forms, or highly complex, as in the gastropods and cephalopods. Fertilization can be either external or internal and eggs may be laid singly or in masses most often covered in a horny material.

The Shell

     Most mollusks inhabit a shell structure that they have secreted from their mantle. The mantle is an extension of the body wall surrounding the visceral mass, which contains shell-secreting glands in those mollusks where a shell is present. The mollusk's shell begins as a calciferous liquid (containing calcium carbonate, CaCO3) that is secreted by the glands situated in the mantle and then hardens. The shell is composed of three layers. The innermost is a shiny layer of nacre, or mother-of -pearl, which is a combination of the calcium carbonate mineral argonite and a protein called conchiolin. The middle layer of the shell is formed of prismatic calcium carbonate crystals. The outer layer, or periostracum, is made of conchiolin.

The Classes

     The phylum Mollusca is the second largest phylum in the animal Kingdom, surpassed in number only by insects. The phylum mollusca is full of diversity. From their size to their shape to their texture to their shell, no two mollusks are identical, but they are all beautiful.

     There are over seventy thousand species of mollusks, and new species are constantly being discovered with the help of todays advanced technoligies. The phylum Mollusca has been divided into six main categories or classes:

class Gastropoda (or Univalve),

class Pelecypoda (or Bivalve),

class Amphineura,

class Cephalopoda,

class Scaphopoda (or Tusks) and

class Monoplacophora.

     These various classes differ in the form of the shell, the structure and location of the foot and the presence, or absence of a head and sensory organs. These classes have been divided into orders, which, in turn, have been divided into families. Each family is then sub-divided into genera which are then divided into species. Members of a species usually look similar and are capable of interbreeding.

Home

Intro

Gastropoda

Pelecypoda

Cephalopoda

Amphineura

Monoplacophora

Bibliography

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