1883.] on Oysters and the Oyster Question. 351 



This substitution is the more interesting since it tends to the same 

 conclusion as that towards which all the special peculiarities of the 

 oyster lead us ; namely, that, so far from being a low or primitive 

 form of the group of lamellibranchiate mollusks to which it belongs, 

 it is in reality the extreme term of one of the two lines of modifica- 

 tion which are observable in that group. The Trigonice, the arks, the 

 cockles, the freshwater mussels, and their allies, constitute the central 

 and typical group of these mollusks. They possess two sub-equal 

 adductors, a large foot, and a body which is neither very deep nor 

 very long. From these, the series of the boring bivalves exhibits a 

 gradual elongation of the body, ending in the ship- worm (Teredo') as 

 its extreme term. While, on the other hand, in the sea mussels, the 

 Avicidce, and the scallops, we have a series of forms which, by the 

 constant shortening of the length and increase of the depth of the body, 

 the reduction of the foot, the diminution of the anterior of the two 

 adductors, and the increase of the posterior, until the latter becomes 

 very large and the former disappears, end in the oyster. 



And this conclusion, that the oysters are highly specialised Lamelli- 

 branchs, agrees very well with what is known of the geological history 

 of this group, the oldest known forms of which are all dimyary, while 

 the monomyary oysters appear only later. 



"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 dejjosits new layers of 

 shell over its whole sui'face, so that the larval shell valves become 

 separated from the mantle by the new layers which crop out beyond 

 their margins and acquire the characteristic prismatic and nacreous 

 structure. The summits 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 

 mould themselves upon it ; and the animal acquires the asymmetry 

 characteristic of the adult. 



Oysters are gregarious, in consequence of the vast multitude of 

 locomotive larvfe which are set free simultaneously ; and which, being 

 subjected to the same influences, tend to settle about the same time in 

 the area to which the swarm drifts. Millions of oysters are thus 

 aggregated together over stretches of the bottom of the sea, at depths 

 of from one or two, to twenty or more, fathoms, and constitute what 

 are known as oyster beds. 



Although oysters live and grow well enough in estuaries, in which 

 the salinity of the water undergoes large variations, according to the 

 state of the tide and the volume of fresh water that is poured in, yet 

 they do not flourish permanently and breed freely in water with less 

 than 3 per cent, of saline constituents. Thus the Baltic is, at pre- 



VoL. X. (No. 76.) 2 A 



