PHYLUM MOLLUSCA 



645 



it two papillae (&) appear, the rudiments of the gills. The larv;i. 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 Pelecjpoda 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 



J i o . 



tongue - shaped : in 

 some cases there is a 

 byssus-glatid posterior 

 to the foot, which se- 

 cretes a mass of horny 

 fibres, the lijssus, 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 



FIG. 530. 



stages in the metamorphosis of Ano, 



sh. shell; sm, adductor muscle; *t> seiise-c 

 cilia. (From Korschelt and Heider's Embryologi 



ystem is well developed, the largest muscles being either 

 wo adductors, which close the shell, and several bands connected 

 with the foot and byssus : the muscles are usually unstnpe 

 ccelome is reduced to a dorsally-placed pericardium. 



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