OYSTERS AND METHODS OF OYSTER-CULTURE. 273 



of difierent character, the outer brownish, with a friable prismatic structure, the 

 inner dense and nacreous. In the larva there is no such distinction, 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 m and the minute vent r 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, vl, the middle of 

 which presents 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 propels the animal, and, 

 in the absence of gills, probably subserves respiration. 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, 1, only is shown in fig. 2) 

 represents 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 throughout with ciliated cells, and the vibra- 

 tion 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, rs ri, 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 efl'ected, as in the adult, by an adductor 

 muscle, av\, the fibers of which pass from one valve to the other. But it is a very 

 curious circumstance that this adductor muscle is not the same as that which exists 

 in the adult. It lies, in fact, in the forepait of the body and on the dorsal side of 

 the alimentary canal. The great muscle of the adult, fig. 3, M, on the other hand, 

 lies on the ventral side of the alimentary canal and in the hinder part of the body. 

 And as the muscles, respectively, lie on ojiposite sides of the alimentary canal, that 

 of the adult can not be that of the larva, which has merely shifted its position; for 

 in order to get from one side of the alimentary canal to the other it must needs cut 

 through that organ ; but as in the adult no adductor muscle is discoverable in the 

 position occupied by that of the larva or anywhere on the dorsal side of the aliment- 

 ary canal, while on the other hand there is no trace of any adductor on the ventral 

 side in the larva, it follows that the dorsal or anterior adductor of the larva must 

 vanish in the course of develoxjmeut, and that a new ventral or posterior adductor 

 must be developed to play the same part and replace the original muscle functionally, 



though not morphologically. 



* * # # * * # 



When the free larva of the oyster settles down into the fixed state, the left lobe of 

 the mantle stretches beyond its valve, and, applying itself to the surface of the stone 

 or shell to which the valve is to adhere, secretes shelly matter, which serves to cement 

 the valve to its support. As the animal grows the mantle deposits new layers of 

 shell over its whole surface, so that the larval shell valves become separated from 

 the mantle by the new layers (plate viii, fig. 3, S), which crop out beyond their 

 margins and acquire the characteristic prismatic and nacreous structure. The sum 

 mits of the outer faces of the umbones thus correspond with the places of the larval 

 valves, which soon cease to be discernible. After a time the body becomes convex 

 on the left side and flat on the right; the successively added new layers of shell mold 

 themselves upon it, and the animal acquires the asymmetry characteristic of the 

 adult.* 



The horny convex shell of the fry (plate viii, fig. 3, L) may be seen, for 

 a considerable time after attachment, at the umbo or beak of the develop- 

 ing shell of the spat (plate viii, fig. 3, 8). The imder or attached valve of 

 the latter at first conforms closely to the .surface to which it has become 



* Huxley, Thomas H. Oysters and the Oyster Question. The English Hlustrated 

 Magazine, London, Oct. 1883 and Nov. 1883, vol. 1, pp. 17-55, and pp. 112-121. 



F. C. R. 1897 18 



