ZOOLOGY STUDY GUIDE - E. O. Nevedomska - 2013

LECTURE 11. PHYLUM MOLLUSCA (Mollusks)

1. General Characteristics of the Phylum Mollusca

Mollusks (soft-bodied animals) are unsegmented, coelomate invertebrates with a soft, unsegmented body divided into distinct regions. Some inhabit marine environments, others live in freshwater, many are terrestrial, and some lead a parasitic lifestyle.

Approximately 113,000 species of mollusks are known (over 600 in Ukraine). This phylum comprises seven classes: Monoplacophora, Polyplacophora (chitons), Aplacophora, Scaphopoda, Bivalvia, Gastropoda, and Cephalopoda. Typical representatives include slugs, swan mussels (Anodonta), mussels, oysters, octopuses, and squids. Their size ranges from a few millimeters to several meters.

Malacology (from the Greek malakion meaning mollusk, and logos meaning study) is the branch of zoology that focuses on The Study of mollusks.

The body of mollusks is typically divided into three regions: the HEAD, visceral mass, and FOOT, and is covered by a muscular mantle unique to these animals. The mantle (from the Greek mantion meaning cloak or cape) is an outer fold of Skin that covers the entire body or a part of it. The mantle cavity is located between the mantle and the body.

The mantle secretes a shell that serves a protective function. The shell consists of three layers: the outer layer (periostracum, or conchiolin layer), the middle layer (pristine/porcelaneous, or prismatic layer), and the inner layer (nacreous, or mother-of-pearl layer). The outer layer is composed of a horn-like organic substance, while the middle and inner layers are made of calcium carbonate. In some species, the shell is partially or completely reduced.

The body cavity is a true coelom (secondary body cavity) formed by a non-metameric residual coelom, which in most forms is represented by the pericardial cavity and the gonadal cavity (Gonads). Most mollusks possess Cytology/cytology/32.html">Smooth Muscle tissue, which results in slow movement. Only cephalopods, which are active and fast-swimming, possess Striated muscle tissue.

The Digestive System of mollusks begins with the Mouth and ends with the anus. The alimentary canal is divided into three sections: 1) the foregut, consisting of the Oral Cavity, Pharynx (into which the ducts of Salivary Glands open, except in bivalves, with secretions containing digestive Enzymes), and Esophagus; 2) the midgut, featuring an expansion—The Stomach—into which the duct of the digestive gland (hepatopancreas, or Liver) opens; 3) the hindgut, terminating in the anus (which opens into the mantle cavity). The enzymes produced by the saliva and liver facilitate the Digestion of major organic food components. Mollusks feed on both PLANT AND ANIMAL matter.

The excretory Organs are the Kidneys, which are modified metanephridia. One end of the Kidney opens into the pericardial cavity (coelom), while the other opens into the mantle cavity.

The respiratory organs are either gills, located within the mantle cavity, or Lungs. The lungs are functionally represented by the walls of the mantle cavity, which are permeated with Blood Vessels.

The Circulatory system is open (blood pours from blood vessels into the body cavity). There is a central organ of Circulation—The Heart, consisting of a ventricle and one or more atria. The Emergence of the heart provides a more efficient Blood Circulation throughout the animal's body.

The Nervous system consists of 3–5 pairs of well-developed ganglia located in various PARTS OF THE body and interconnected by nerve cords. Nerves extend from them to different body parts. This type of nervous system in mollusks is known as a scattered-ganglion type (Fig. 32).

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Fig. 32. Nervous system of mollusks:

1 — nerve ganglia; 2 — nerve trunks.

The Development of Sense Organs depends on the habitat conditions of mollusks. They possess eyes, organs of Touch, chemical sense (osphradia), and equilibrium (statocysts), which are developed to varying degrees among representatives of different classes.

Mollusks reproduce exclusively sexually. They are either dioecious or hermaphroditic. Fertilization is predominantly internal, though external in some species.

The development of mollusks is either direct or involves metamorphosis. Viviparous forms are also found among them.