XII PHYLUM MOLLUSCA 693 



beset with spines (Fig. 573, sh). At the same time the body of 

 the larva, which has hitherto been an undivided mass projecting 

 between the two valves of the shell, becomes cleft from below up- 

 wards, and thus divided into a single dorsally-placed body proper, 

 and paired — right and left — mantle-lobes. Upon the latter 

 peculiar brush-like sense-organs make their appearance, and on 

 the ventral surface of the body is formed a glandular pouch, which 

 secretes a long thread, the provisional hyssus (/). The mesoderm 

 cells give rise to a single immense Adductor muscle (sm), the fibres 

 of which extend from valve to valve. 



The larva is now called a glochidium : it remains in the brood- 

 pouch, nourished by a secretion from the walls of the latter, and 

 entangled with its fellows by means of the byssus. At this stage 

 the outer ' gill-lamina appears as if stuffed full of closely 

 aggregated sand-grains. Before long the larvae are ejected 



/('( 



B. L 



- sh. 



s^ "7 m 



\7Wr 





■5. 



. sm. 



ffi. '- 



Fit;. 573. —A, advanced embryo of Anodonta. B, free glochidium ; /, provisional byssuM 

 s, shell ; sh, hooks ; sm, adductor muscle ; so. sense-organs ; w, cilia. (From Korsehelt and 

 Heider's E-inhi-yoiogy.) 



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 become attached 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 

 membrane of the host, and are nourished by its juices absorbed 

 thi'ough processes of the mantle. They thus lead a truly ecto- 

 parasitic existence for about ten weeks. 



While in this condition a metamorphosis takes place. The pro- 

 visional byssus and sense-organs disappear (Fig. 574), and imme- 

 diately posterior to the former an invagination, the stomodceum (m), 

 is formed, and soon communicates with the archenteron. 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 development of a proctodseum. The foot (/m) arises 

 as a median ventral elevation behind the mouth, and on each side of 



