CLASS V. PELECYPODA, Goldfuss. 



(= Acephnlatestacea,C\iviei: Conckifera, Lamarck. Lamellibranchia, Blain- 

 ville. Lipocephala, Ray Lankester.) 



AQUATIC, bilaterally symmetrical, acephalous molluscs, protected by 

 a pair of shelly valves, which are secreted by the lateral portions of 

 the mantle, connected by a ligament, and moved by the contiactions 

 of muscles attached to the inner faces of the valves ; feeding by ciliary 

 action, and destitute of a radula or jaw ; breathing by lateral gills ; 

 imperfectly sensible to light, and rarely provided with peripheral 

 visual organs ; possessing olfactory organs (osphradia), auditory and 

 equilibrating organs (otocysts), tactile papilla?, and a nervous system 

 composed of ganglia united by nerves, but without a pedovisceral 

 commissure ; provided with an extensile, tactile, or locomotor organ 

 (foot) ; a circulator}' system containing htemolymph, and operated 

 by a single or paired cardial ventricle and 2 auricles ; a more or less 

 convoluted intestinal canal, with its oral and anal extremities at 

 opposite ends of the body ; a stomach ; paired nephridia, connected 

 with the pericardium, and discharging independently of the rectum ; 

 reproducing without copulation, by eggs and spermatozoa ; monoecious 

 or dioecious ; development external to the ovary ; the post-larval 

 stage protected by a prodissoconch, and sometimes exhibiting a special 

 nepionic stage. (Dall.) 



All the Pdecypoda are aquatic. The great majority are marine, 

 but some few families have penetrated into fresh waters. All the 

 members of the class feed upon microscopic organisms, chiefly Diatom- 

 acece and other low forms of plant-life. Only the Septibranchia and 

 some other abyssal forms are truly carnivorous. 



In general, the Pelecypods are burrowing forms, living half -buried 

 in muddy or sandy bottoms, and in this case their plane of symmetry 

 is vertical. But many forms are completely sedentary, and are fixed 

 by the byssus, or in a more definite manner by the shell itself, as is 

 the case in Spondi/lus, Ostrea, &c. In these genera the plane of sym- 

 metry becomes horizontal, and the animal usually lies on the right 

 side e.g., Spondylus, Anomia ; more rarely on the left side, as in Ostrea. 

 Some Pelecypods live in holes which they excavate either in wood, 

 as in the case of Teredo, or in stone, as Lithophaga, Saxicava, Phola- 

 didea, &c., or even in the shells of other molluscs. 



Some Pelecypods, such as Lima, are nidamentous, and construct 

 a nest by means of the byssus. Others live in the tests of Ascidians, 

 in sponges ; the few commensalistic forms generally live on or in 

 Echinoderms ; a few are commensal with Crustaceans. 



