XII 



PHYLUM MOLLUSCA 



645 



it two papillae (&) 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- 



R 





/f, 



B. 



\ 



careous shell. There ^/ f 



is no distinct head. 

 The ventral region of 

 the body is differenti- 

 ated into a muscular 

 foot, which is usu- 

 ally ploughshare- ox- 

 tongue - shaped : in 

 some cases there is a 

 byssus- gland posterior 

 to the foot, which se- 

 cretes a mass of horny 

 fibres, the lyssus, by 

 which the animal may 

 be permanently at- 

 tached. There are 

 usually two pairs of 

 gills, but the two gills 

 of each side are to be 

 looked upon as modi- 

 fications of a single 

 primitive gill or cteni- 

 dium : the chief func- 

 tion of the gills is the 

 production of a re- 

 spiratory 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 developed, the largest muscles being either one or 

 two adductors, which close the shell, and several bands connectedj 

 with the foot and byssus : the muscles are usually unstriped. The 

 ccelome is reduced to a dorsally-placed pericardium. The mouth 



T T** 



IV. 



y 



fu. 



FIG. 536. Three stages in the metamorphosis of Anoy 

 donta. d, enteric canal; /, provisional byssus; .Ac- 

 foot ; g, lateral pits ; A-, rudiments of gills ; m. mouth ; 

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

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





