1. Dr. Sanjay Kumar Jha Department of Zoology P.G.G.C.G. – 42, Chandigarh

2. The phylum Mollusca are next to Arthropoda as regards the number of genera, species and the diversity of form, size and organization. This phylum is commonly represented by mussels, oysters, snails, slugs, squids and cuttlefishes, etc. possessing the following characteristics:-  BODY FORM: The body has a variety of shapes and is usually unsegmented. Neopilina is a segmented mullusk and is connecting link between phylum Annelids and Mollusks.  SYMMETRY: Their body has a bilateral symmetry, but some forms get secondarily twisted due to torsion during growth and become asymmetrical.  GERM LAYERS: Mollusks develops form three germ layers, viz., ectoderm, mesoderm and endoderm, hence triploblastic animals.  LEVEL OF ORGANIZATION: The mollusks have definite organ-systems for various metabolic activities, hence, possess organ-system level of body organization.  BODY PARTS: The body is generally soft and is distinguished into three regions, viz., ant. Head with sense organs, dorsal Visceral mass having organ-systems and ventral muscular Foot for locomotion. Foot is modified for creeping, burrowing, swimming, etc.

3.  BODY WALL: Epidermis is one-layered, usually ciliated, with mucous glands. Muscles are unstriped or unstriated and occur in bundles.  MANTLE (PALLIUM): A thin, fleshy fold or outgrowth of dorsal body wall covers the body (fully or partially) and is called the mantle or pallium. It encloses a space called mantle or pallial cavity, between itself and the body.  SHELL: The mantle usually secretes an external hard and calcarious Shell of 1, 2 or 8 pieces. The shell may, however, be internal, reduced and even absent in some mollusks.  BODY CAVITY: Coelom is greatly reduced and is represented by cavities of pericardium, kidneys and gonads. Spaces among the viscera contain blood and form haemocoel.  DIGESTIVE TRACT: The digestive tract is complete. Buccal cavity often contains a rasping organ, the Radula with transverse rows of teeth. Anus opens into the mantle cavity. A large digestive gland, the Hepatopancreas, functions as a digestive, absorptive and storage organ.  RESPIRATION: Respiration usually takes place by one to many lamellate or plume-like gills called the Ctenidia. Respiration also occurs by body surface, mantle or a pulmonary sac (both aerial and aquatic respiration).  CIRCULATORY SYSTEM: Circulatory system is Open type and includes a dorsal, pulsatile heart, surrounded by pericardial cavity. Blood contains a Cu 2+ containing respiratory pigment, Haemocyanin, dissolved in plasma.

4.  EXCRETORY SYSTEM: It includes 1 or 2 pairs of metanephridial tubules called Kidneys, which communicates with the coelom (pericardial cavity) on one side by a nephrostome and open into the mantle cavity on the other side by a nephridiopore. They, thus have the nature of coelomoducts. Waste matter is chiefly Ammonia in aquatic forms and Uric acid in land forms.  NERVOUS SYSTEM: The nervous system typically comprises of three paired ganglia – Cerebral (above the mouth), Pedal (in the foot) and Visceral (in the visceral mass), all interconnected by commissures and connectives. Sense organs include tentacles, eyes, statocysts for equilibrium and ospharidium for testing the chemical and physical nature of water.  REPRODUCTION: Sexes are usually separate, united in some. Gonads usually vary from 1 to 2 and have gonoducts. Fertilization may be external or internal. Most forms are oviparous (few are viviparous). Asexual reproduction is lacking.  DEVELOPMENT: Cleavage is usually spiral, determinate and unequal. Life history may be direct or with a larva, first Trochophore then Veliger (modified trochophore).  NATURAL HISTORY: Mollusca are predominantly marine animals. Many, however, occurs in fresh water and some even on land. There are over 50,000 living species of mollusks. In addition, some 35,000 fossils species are also known. The mollusks are precambrian in origin.

5.  Three body regions, viz., anterior head, dorsal visceral mass and ventral foot.  A glandular fold, the mantle, over the body.  A space, the mantle cavity, present between the mantle and the body and having apertures for anal, excretory and reproductive systems.  External calcareous shell in most forms.  A rasping organ, the radula, in the buccal cavity of most species.  Sac-like kidneys, opening into the coelom at one end and into the mantle cavity at the other end.

6. a)Body divided into 2 or 3 regions in all cases. b)A distinct head with much better sense organs in most cases. c)A muscular locomotory organ, the foot, in all cases. d)Unstriated muscles arranged in bundles for moving specific parts of the body. e)Gills and lungs for respiration in nearly all forms. f)Heart comprising 1 to 2 auricles and a ventricle. g)No asexual reproduction.