62 COMMISSION OF CONSERVATION 



to connect with each other posterior to or below it ; and then all four palps would have 

 to be turned backwards, flattened out, and have the base of attachment extended some 

 distance along the sides of the abdomen; the surfaces of the two palps of each side turned 

 towards each other would have to become ridged, perhaps along tertiary folds of the 

 velum, and the cilia become increased and re-distributed along these ridges, while the 

 other surfaces of the palps would become plane. Thus the velum would have to be- 

 come relatively reduced, remodelled, changed in position, attachment, folding and 

 ciliation. Moreover there must be a more intimate relation of the ciliated epithelium 

 of the palps to that of the inside of the mouth and oesophagus than exists between the 

 velum and the mouth, although when the shell is gaping and the larva apparently feed- 

 ing, the ciliation of the median portion of the posterior half of the velum seems to act 

 towards and into the mouth, and I am inclined to think that the very act of swimming 

 itself tends to sweep diatoms and other food particles into the mouth. I have sections 

 showing long cilia of the anterior face of the folded velum pointing backwards into the 

 mouth, and I have noticed in living larvae the posterior part of the rim of the mouth 

 projected forwards below, so that the hollow vertical crease of the velum led back into 

 the open oesophagus. From a consideration of the size of the velum in the larva and 

 the size of the smallest recognizable palps in the spat it would appear that the velum 

 would have to suffer a period of decay, followed by a period of vigorous growth. Re- 

 stricting one's observations to the oyster it might seem more likely that the lower palps 

 should originate from the foot and only the upper ones from the velum. The foot is 

 ciliated and has a median ventral furrow corresponding with the division-line between 

 the lower palps. In that case we would expect to find supra-cesophageal ganglia in 

 the bases of the upper palps, in front of the oesophagus, as well as vestiges of pedal 

 organs (pedal ganglia, otocysts) in the bases of the lower palps, behind the oesophagus. 

 But in my sections I have not been able to recognize these structures, and reflection on 

 other bivalves, where both foot and palps persist as functional organs in the adult 

 (clam, mussel, &c), proves beyond doubt that the foot has nothing to do with the 

 lower palps. 



In the face of the above mentioned difficulties it may be questioned also whether 

 the velum has anything to do with the palps. In some cases larvae with the velum pro- 

 truded and partly severed from the body may be seen, as well as completely separated 

 vela, still capable of muscular and ciliary swimming movements. This suggests the 

 possible atrophy and dehiscence of the organ, such as was asserted by Davaine but de- 

 clared by Ryder as not having been confirmed by any other observer. I can confirm 

 the observation, but I do not regard it as conclusive. Such cases may result from the 

 accidental pinching off by a snapping closure of the shell- valves. The small size, shrun ken 

 appearance, enfeebled movements, and even the accident itself point towards an ante- 

 cedent loss of ability to respond corrclatively to the activity of other organs such as the 

 contraction of muscles and closing of the shell. In certain free-swimming larvse, that 

 may be old and perhaps belated in their efforts to become attached, the velum can be 

 observed to suffer reduction in size and vigour. This would seem to point to a natural 

 decay of the organ, that might be followed by complete resorption. 



In this connection may be mentioned the young spat obtained and figured by Jack- 

 son (1890, Plate XXIV, figs. 1, 2). He states "A large, lobed, ciliated velum stillexists. 



The long vibrating cilia were in active motion and some motion of the velum 



as a whole was noticed; but it was not seen to extend beyond the margins of the shell." 

 This is the stage upon which Jackson's original work began. If he had known the free 

 larva immediately preceding fixation he would have recognized at once that the velum 

 in his spat was not "large" but reduced, and doubtless also that the activity of the 

 velum and of the cilia was likewise becoming arrested. In fact he did ^Bfcnotice that 

 in 24 hours the velum had disappeared. 



Of the thousands of larva: at one's disposal in plankton it may occur 

 that relatively few show a disposition to swim, creep, or otherwise exhibit 

 their soft parts in activity. Under such conditions it is not safe to build 

 too much on the small number of observations of dehiscence or of re- 

 duction. I have therefore attempted to make observations on what I 

 felt sure were normal and healthy individuals, immediately before, at, 

 and closely after fixation. The size, depth, overlying of organs, presence 

 of pigment, absence of movements under the unnatural conditions of a 



