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Class CONCHIFERA. 



Head indistinct; mouth with elongate fleshy lips or palpi. 

 Body covered -with a hilobed mantle, each lobe protected by 

 a shelly valve. Gills lamellar, two on each side. Foot 

 placed under the body, usually compressed and keeled. 

 Valves of shell united on their dorsal edges by a ligament. . 

 Animal aquatic. Individual bisexual. 



The Bivalve Mollusca are without apparent head, and 

 have neither eyes nor tentacles, and the foot, although at- 

 tached to the belly, does not form a flattened disk as in the 

 Gasteropods, but is laterally compressed, allowing these 

 animals to leap or burrow, but rarely serving them as an 

 organ of reptation. The mantle, which envelopes their 

 body, encloses generally a pair of gills on each side, and 

 is protected by shelly valves ; the valves, however, are not 

 usually spiral, as in the Gasteropoda, but are simply con- 

 cave, and joined together at the hind part by a horny liga- 

 ment. 



The Conchifera are strictly hermaphrodite and self- 

 impregnating, having both sexes combined in the same 

 individual. They are oviparous, and their fecundity is 

 generally very great, a single Unio having been estimated 

 to produce three hundred thousand young in one season. 

 The eggs are hatched in the gill-cavity, or in the outer 



