CLASS ACEPHALA. 



35 



FIG. 48. 



CLASS ACEPHALA (a sef'a la). 



The ACEPHALOUS (a sef'a lus) (headless) MOLLUSKS have 

 the sides of the body protected by a pair of shells, or 

 valves, from which peculiarity they are 

 sometimes called Bicalccs (bi'valvz).* 

 These valves are united by a hinge, 

 and holding them apart is an. elastic 

 ligament (lig'a merit), which, if placed 

 inside, acts by expansion ; if outside, 

 by contraction. The valves are closed 

 by one or two bundles of muscular 

 fibers, which the fisherman severs, be- 



Diagrammatic section 



fore he can take out the soft parts. through a ciam, to nius- 



mi ,1 -i 111 trate the opening and clos- 



The mouth may be surrounded by a 



series of tentacles, f to direct the course 



of the food, which is induced, by ciliary 



movement, to enter with the water 



into the mantle cavity. The gills are 



large and lamellate (lam el'iat), a characteristic which has 



given rise to the name LameUibranch da mei'll brangk). 



The so-called "foot" is a development of the lower lip, 



and is of use in burrowing and in locomotion.^ All 



Acephalous Mollusks are aquatic. 



ing of the valves. L, Elas- 

 tic ligament, which con- 

 stantly tends to separate 

 the valves. M, Adductor 

 muscle, which, by an act of 

 volition, brings the valves 

 together. 



ORDER ASIPHONIA (a Si fon'l a). 



The common Oyster of the Atlantic coast, like other 

 members of the class, on hatching from the egg is free 

 to swim about ; but soon attaches itself to some support 



* These are not homologous with those of the Brachiopods, as they ar<- awiinst 

 the sides of the animal, not above and below. 



t Compare with mouth tentacles of Polyzoa, page 33. 



% Compare with the creeping surface of Planarian, page 2.5. 



