694 



ZOOLOGY 



SECT. 



it two papilla (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. 



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- 

 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 

 byssus-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 coelome 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 ; fn, 

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

 sh. shell ; sin, adductor muscle ; so. sense-organs ; w, 

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



