66 COMMISSION OF CONSERVATION 



cretion as a tapering point which falls over, making a hook-like or knob-like 

 process. 



Succeeding stages show that the gland suffers rapid degeneration 

 after the completion of its work. Later and larger stages of the spat must 

 increase the surface of attachment by a different method, viz., by the same 

 means and at the same time as the growth of the shell. 



Ryder (1882 p. 283) attempted to discover the way in which the larva attaches 

 itself, and was misled by confining his attention to straight-hinge stages, adhering as 

 nearly as he could make out by the edge of the protruding mantle. He supposed that 

 this was the normal manner of first becoming adherent, and that afterwards the at- 

 tachment was made firm by the building of a few layers of shelly matter along the edge 

 of the lower valve. 



Horst (1884, p. 907) also observed a little band of secondary shell substance, secreted 

 along the edge of the shell of the larva, which he says may possibly have aided the 

 little oyster in adhering, but regrets that he was unable to solve the problem satis- 

 factorily. 



Huxley (1883, p. 113) in like manner states: "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." 



Rice (1883 p. 28) wrote: "Further observation seems to show that this is their 

 normal mode of attachment, that is, to thrust out the velum from between the shells 

 and adhere to whatever is within reach, afterwards the animal falls over to one side, 

 generally the left, and the shell of that side gradually forms around and out beyond 

 this attachment of the young animal." 



Jackson (1900 p. 303) says that "The preliminary fixation is probably effected 

 by means of the reflected mantle border, as described by Ryder, and is then immedia- 

 ately succeeded by a cementing conchyolin attachment of the extreme edge of the lower 

 left prodissoconch valve." 



Regarding the presence of a byssus, Ryder ('82 p. 383; '82-'83, p. 329; '84 p. 758) 

 was doubtful. Horst believed that he had noticed a small byssus. 



Jackson ('90 p. 302-3) reasoned that "As the byssus is an organ developed in the 

 ventral portion of the foot, the high reduction or almost complete absence of that 

 organ is in itself strongest evidence against the suggestion that the attachment of the 



young oyster is effected by means of a byssus of however short a duration In 



view of the evidence it is therefore safe to conclude that the oyster does not have a 

 byssus at any period of its development." 



The Mantle, in living spat, may often be seen protruding beyond the 

 edges of the shell-valves, but in preserved specimens it is retracted, some- 

 times close up to the gills and body, so that the soft parts of the animal 

 may occupy a half to a third of the cavity of the shell. In even the young- 

 est stages the margins of the mantle are thickened and have longitudinal 

 grooves and short tentacular processes (Plates VI and VII). It is these 

 thickened margins that contain numerous unicellular glands, doubtless for 

 secretion of the substances of the shell. Under the hinge there is a special 

 pad of gland cells for secretion of the hinge-ligament. Differences in the 

 two halves of the mantle afterwards become noticeable, such as thickness, 

 length, and the greater freedom of the right side, where, for a considerable 

 area in front of the adductor muscle, there is open communication from 

 the supra-branchial chamber to the outside at the dorsal edges of the 

 mantle (Plate VII, fig. 14). Anteriorly the right and left margins grow 

 together for a distance, making a sort of hood in front of the mouth. 



