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Abstract
The freshwater bivalve Caelatura teretiuscula has an elongated shell. The two shell valves hinged together by hinge-ligament and cardinal & lateral teeth. It is covered internally by two mantle lobes under which the soft parts are found. The inhalant and exhalant siphons are formed from the posterior edges of the mantle lobes.
The digestive system of the present clam consists of the mouth, oesophagus, complicated stomach, combined style sac and midgut, intestine, rectum and digestive gland.
The sexes are separated in C. teretiuscula and the reproductive system consists of one pair of large gonad occupying most of the visceral mass behind the digestive gland. The gonad composed of large number of follicles lead to numerous small ducts open into the two gonoduct. During reproductive season, each follicle contains all different stages of sex cells.
Electron microscopic examination shows that. spermiogenesis passes through three stages; early, mid and late spermatids. Mature sperm is of a primitive type with bullet- shaped nucleus and five mitochondria. The sperm head characterized by the absence of acrosomal structure.
Key words:
Bivalves, Unionidae, Morphology. Anatomy, Histology, Histochemistry, Digestive system, Reproductive system, Spermiogenesis