694 



ZOOLOGY 



it two papillae (k) appear, the rudiments of the gills. The larva is 

 now fitted for free existence ; it drops from its host, and gradually 

 assumes the adult form and mode of* life. 



so. 



2. Distinctive Characters and Classification. 



The Pelecypoda are bilaterally symmetrical, compressed Molluscs, 

 in which the mantle consists of paired right and left lobes, secret- 

 ing a bivalved cal- 

 f careous shell. There 



is no distinct head. 

 The ventral region of 

 the body is differenti- 

 ated into a muscular 

 foot, which is usu- 

 ally ploughshare- or 

 tongue - shaped : in 

 some cases there is a 

 hyssus-gland posterior 

 to the foot, which se- 

 cretes a mass of horny 

 fibres, the byssus, by 

 which the animal may 

 be permanently at- 

 tached. There are two 

 gills or denidia, one 

 on each side : the chief 

 function of the gills is 

 the production of a 

 respiratory and food- 

 carrying current of 

 water. The body is 

 covered by a one- 

 layered epidermis, 

 which is ciliated on 

 the gills and on the 

 inner surface of the 

 mantle. The muscular 

 system is well-de- 

 veloped, the largest 

 muscles being either 

 one or two adductors, 

 which close the shell, 

 and several bands connected with the foot and byssus ; the 

 muscles are usually unstriped. The ccelome is reduced to a 

 dorsally-placed pericardium. The mouth is bounded by two pairs 

 of flat, triangular tentacles or labial palps, the cilia of which 



Fig. 574. — Three stages in the metamorphosis of Ano- 

 donta. d, enteric canal ; /. provisional byssus ; fu, 

 foot ; .</, lateral pits ; k, rudiments of gills ; m. mouth ; 

 sh. shell ; sm, adductor muscle ; so. sense-organs ; ie, 

 cilia. (Prom Korschelt and Heider's Embryology.) 



