780 KEPOET OF COMMISSIONER OF FISH AND FISHERIES. 



[2] 



oyster. In the adult the shell is composed of two substauces of differ- 

 ent character, the outer brownish, with a friable prismati(5 structure, 

 the inner dense and nacreous. In the larva there is no such distinc- 



FlG. 1. 



tion, and the whole shell consists of a glassy substance devoid of any 

 definite structure. 



"The hinge line answers, as in the adult, to the dorsal side of the body. 

 On the opposite or ventral side, the wide mouth o and the minute vent a 

 are seen at no great distance from one another. Projecting from the 

 front part of the aperture of the shell there is a sort of outgrowth of 

 the integument of what we may call the back of the neck, into a large 

 oval thick-rimmed disk termed the velum, v, the middle of which pre- 

 sents a more or less marked prominence. The rim of the disk is lined 

 with long vibratile cilia, and it is the lashing of these cilia which pro- 

 pels the animal, and, in the absence of gills, probably subserves respi- 

 ration. The funnel-shaped mouth has no palps; it leads into a wide 

 gullet, and this into a capacious stomach. A sac-like process of the 

 stomach on either side (the left one II only is shown in Fig. 1) repre- 

 sents the 'liver.' The narrow intestine is already partially coiled on 

 itself, and this is the only departure from perfect bilateral symmetry in 

 the whole body of the animal. The alimentary canal is lined through- 

 out with ciliated cells, and the vibration of these cilia is the means by 

 which the minute bodies which serve the larva for food are drawn into 

 the digestive cavity. 



" There are two pairs of delicate longitudinal muscles, rsri, which are 

 competent to draw back the ciliated velum into the cavity of the shell, 

 when the animal at once sinks. The complete closure of the valves is 



