Introduction:

The phylum mollusca are quite difficult to state as many of living groups bear very little similarity to each other. Their association can only be traced by fossil records that, unluckily as they are soft-bodied, are not eagerly available. They are unsegmented and contain well-developed muscular locomotory organ, foot, and fold of dorsal wall known as a mantle; the majority of them also have exoskeleton in form of a calcareous shell secreted by mantle. Most have a specialized feeding organ (radula). In other ways, molluscs follow higher invertebrate plan in being triploblastic, with bilateral symmetry and with all organ systems. Coelomic cavity is decreased and frequently replaced by the system of blood sinuses. Aquatic forms have gills for respiration.

The basic body plan of mollusks:

• The descriptions of features of hypothetical ancestral mollusc are as follows:
• It was believed to have lived in shallow pre-Cambrian seas, crawling over rocks and feeding on algae.
• Small in size, bilaterally symmetrical, oval in shape
• Shell was low, conical and a little more than tough cuticle.
• Clamped down on rock surface, covering body as defence against predators
• Crawled by means of muscular foot with ciliated cells and mucous glands on its ventral surface
• Salivary glands generate mucus which both lubricates radula and entangles food particles.
• Anterior part of stomach, style sac, contains a ciliated lining, with cilia all beating at right angles to its longitudinal axis causing strings of food and mucus to rotate, to form central mass known as the protostyle.
• The alimentary canal is completed by long intestine, in which faecal pellets created are swept away by respiratory current.
• Kidneys of ancestral mollusc were probably like annelid nephridia, with nephrostome, tubular duct, and nephridiopore.
• Present day molluscs have the open blood system, coelom being limited to pericardial and perivisceral structure. The typical molluscan heart consists of a ventricle with a pair of auricles opening into it.

Basic and special characteristics of phylum Mollusca:

• Body more than 2 cell layers thick, with tissues and organs and Bilaterally symmetrical.
• Without a body cavity other than provided by blood sinuses.
• Body monomeric and very variable in form, but essentially squat and frequently conical, elongated in dorsoventral plane to form 'visceral hump'; basically with anterior head bearing eyes and sensory tentacles, large flat ventral foot, and posterior mantle cavity.
• Protective, external dorsal shell of protein (conchiolin) reinforced by calcareous spicules, or from one to eight calcareous plates, secreted by the dorsal and lateral epidermis (the mantle); at times shell secondarily decreased, covered by tissue, or lost, and at times enlarged so as to cover whole body
• Toothed, chitinous, tongue-like ribbon, radula, can be protracted from buccal cavity through mouth for feeding.
• The open blood system (blood doesn't flow through the continuous system of discrete vessels but by blood spaces surrounding organs) and heart enclosed within mesodermal cavity, pericardium, by which intestine also passes.
• Pair of sac-like 'kidneys', open proximally in pericardium and discharging in mantle cavity.
• Nervous system with the circum-oesophageal ring and 2 pairs of ganglionated longitudinal cords, at times highly concentrated.

Diversity of molluscs:

With more than 100,000 named species separated between 8 classes, phylum Mollusca ranks second to phylum Arthropoda.

Classes and Characteristics: Categorization of molluscs is based on features of foot, shell, mantle, radula and respiratory organs; based on the features, there are 8 classes as follows:

• Chaetodermomorpha (Aplacophora)
• Neomeniomorpha (Aplacophora)
• Monoplacophora
• Polyplacophora
• Scaphopoda
• Gastropoda
• Bivalvia
• Cephalopoda

Class Chaetodermomorpha (solenogasters; Aplacophora):

70 species are known. They live head-down in the burrows, in soft marine sediments. Posterior end bears a pair of ctenidia that are placed at entrance of burrow. They are Worm-like, 2mm-14cm long, Lack foot, and shell (spicules in place of the shell), movement by peristaltic contractions, mantle covers whole body, head is badly developed/reduced (no tentacles, no eyes), they contain radula and style sac. Like Neomenia sp., Chaetoderma sp., etc.

Neomeniomorpha (solenogasters; Aplacophora):

Approx 180 species are known. They look like chaetodermomorphs. They are Worm-like and elongated. They contain no shell, essentially headless, no radula in some. They vary from Chaetodermomorpha as follows: 1mm-30cm long, body laterally compressed, ventral groove, greatly reduced foot, mantle covers body; 1 or more layers of calcareous scales are embedded in mantle. Movement is through gliding. All species are hermaphrodites. Copulation occurs with the aid of stylets.

Class Monoplacophora (single plate):

They are circular in shape and bilaterally symmetrical. They resemble limpets but their shell is tipped forward. The shell is made from one unit with definite growth rings. The foot is well developed, broad and flat and disc-shaped. They have internal metamerism (the only segmented molluscs). Their nervous system lacks ganglia E.g. Neopilina

Class Polyplacophora (bearer of many plates; Chitons; Amphineura):

Their body is oval, bilaterally symmetrical and dorso-ventrally flattened. Foot is flat and occupies majority of the ventral surface of body. Mantle forms the thick girdle around foot. Shell is composed of several units (usually 8). There are ctenidia in 2 rows, one on either side of body. Head is reduced devoid of eyes or tentacles. Fertilization is external. trochophore larva grows to adult without passing by veliger stage. E.g. Chiton

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