67(3 



ZOOLOGY 



SECT. 



through the exhalant siphon, and if they happen to come in 

 contact with a passing Stickleback or other fresh- water fish, 

 fix themselves on some part of its body by means of the hooked 

 valves. The glochidia of Unio usually attach themselves to the 

 gills, those of Anodonta to the skin or the fins. In this position 



they become encysted by 

 an overgrowth of the 

 skin or mucous mem- 



brane of the host, and 

 are nourished by its 

 juices absorbed through 

 processes of the mantle. 

 They thus lead a truly 

 ectoparasitic existence 

 for about ten weeks. 



While in this condi- 

 tion a metamorphosis 

 takes place. The pro- 

 visional byssus and 

 sense-organs disappear 

 (Fig. 584), and immedi- 

 ately posterior to the 

 former an invagination, 

 the stomodceum (m), is 

 formed, and soon com- 

 municates with the arch- 

 enteron. The posterior 

 end of this cavity is in 

 close contact with the 

 ectoderm, so that the 

 anus is formed by a 

 simple process of rupture, 

 and without the develop - 

 ment of a proctodaeum. 



FIG. 584. Three stages in the metamorphosis of Ano- The foot (fu) arises as a 



donta. d. enteric canal ; /. provisional byssus ; fu. Tr)pr li ar , vpnfral pWafinn 

 foot ; g. lateral pits ; k. rudiments of gills ; m. mouth ; 



sh shell ; an. adductor muscle ; so. sense-organs ; w. behind the mouth, and On 

 cilia. (From Korschelt and Heider a Embryology.) , . , . . ' 



each side 01 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. 



2. DISTINCTIVE CHARACTERS AND CLASSIFICATION. 



The Pelecypoda are bilaterally symmetrical, compressed Molluscs, 

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

 a bivalved calcareous shell. There is no distinct head. The 

 ventral region of the body is differentiated into a muscular foot, 



