ORGANS OF THE SPAT 61 



which originally called it into existence (csenogenesis) . There may have been an ad- 

 vantage in the gradual shifting^ of the asymmetry from post-larval to larval stages of 

 the offspring, such as determining that the heavier side, when swimming or creeping 

 movement ceases, should promptly come in contact with the point of fixation and at the 

 same time insure the most favourable relative positions for the activity of organs whose 

 functions had been perfected under like conditions in more adult stages. 



The temporary transfer of the greater convexity to the upper valve in young stages 

 of the spat can be explained by the fact that for a time the growth of the lower valve 

 follows, as it is cemented to, the surface upon which it rests, wliile the upper valve is free 

 for growth in every direction. When sufficient svuface of attachment is secured the 

 edge of the lower valve becomes free, and then the greater convexity soon reverts to 

 the left valve again. 



Two larval locoinotory organs soon disappear under the new con- 

 ditions brought about by fixation, viz., the velum and the foot. 



The Velum, it has been thought, may become converted into some 

 other organ (palps or gills), or it may become lost by dehiscence. 



"It has been suggested by Loven, though without any direct evidence, that the 

 labial tentacles of adult Lamellibranchiata are the remains of the velum. The velar 

 area is in any case the only representative of the head." (Balfour 1880, p. 215). 



" Davaine makes the statement that the velum appears to drop off some time about 

 the end of the larval period. Gerbe, on the other hand, asserts that the velum is 

 transformed into the palps" (Ryder 1882, '84, p. 790). 



De la Blanchbre thought it probable that the gills originated in the velum (Horst 

 1884, '86, p. 896). 



Ryder says : " The detachment of the ring or crown of vibratory filaments or cilia 

 from the enibrvo as asserted by Davaine has not been confirmed by any other observer" 

 (1882, p. 404).' 



From the straight-hinge larva, such as was the only stage known to the above men- 

 tioned observers, to the fully formed spat or oyster, possessing palps and gills, was a 

 long jump; yet the presence of a velum in the first and its absence in the last, coupled 

 with the absence of palps in the first and their presence in the last, together with their 

 anterior position and close association with gills of very similar appearance, was sufii- 

 cient, in the absence of any more definite information, to suggest the connection. If 

 De la Blanchere had been acquainted with the older umbo-stages of the larva he would 

 have known that velum and gills exist at the same time, which would have disposed of 

 the possibility of one being converted into the other. 



The view of Lov^n and of Gerbe, that the velum is destined to give rise to the labial 

 tentacles (palps), and that therefore it does not become entirely useless and completely 

 disappear, is a fascinating one, that seems to be supported by several bits of evidence. 

 It is, however, not easy to picture to one's self exactly how the transformation 

 might come about. It must be remembered that the velimi is in front of 

 the oesophagus, while the palps are behind it. Davaine seemed to think 

 that the mouth perforated the disc of the velum, in which case matters 

 would have been easier. The two anterior, outer or upper palps, forming 

 a pair, are connected by a rim around in front of the mouth or open end of the 

 oesophagus, thus constituting a sort of upper lip. In like manner the two posterior, 

 inner or lower palps form a pair and connect by means of a lower lip behind the mouth. 

 In the larva (Plate V, fig. 31) there is a minute free rim to the outer end of the oesopha- 

 gus, often flattened out as if absent in front, the rest of the oesophagus being bound up 

 wit la the median part of the posterior surface of the velum. When the velum is retracted 

 the oesophagus occupies a somewhat vertical position, curving gently backwards and 

 then forwards, and the mouth is full length of the oesophagus below the stomach and 

 full depth of the velum below the anterior adductor muscle. When the velum is pro- 

 truded the oesophagus comes to be stretched out in front of the stomach, and the velum 

 is now above the oesophagus, but still separating the mouth from the anterior adductor 

 muscle. In the spat, after the disappearance of the velum, the mouth comes to be fixed 

 in front of the stomach, in a position just behind where the anterior adductor muscle used 

 to Ije. During the metamorphosis of the larva into the spat the oesophagus would have to 

 rnove broad-front up through the velum, splitting it in the median sagittal plane into 

 right and left halves along its main vertical line of folding; each half would then have 

 to become secondarily folded to give rise to the anterior and posterior palps of its own side; 

 the posterior palps of opposite sides would have to grow around the oesophagus in orde r 



