1876.] 231 [Brooka. 



face of the ocean, by means of the long cilia of the velum, which 

 would seem to perform the function of a respiratory organ as well, 

 for the fluid which fills the body-cavity is driven into and out of the 

 sinuses of the velum by the retraction and expansion of this struc- 

 ture ; in most veligers this circulation seems to be aided by the 

 rythmical contraction of the muscular fibres which bind the foot to 

 the oesophagus. The mouth is not within the circlet of large locomo- 

 tive cilia, but immediately behind it, and a ring or band of smaller 

 cilia passes from the anterior margin of the mouth entirely around 

 the velum, on its lower surface, and therefore outside the circlet of 

 locomotive cilia. This second circlet seems adapted to convey food 

 to the mouth, but there are no direct observations upon this point. 

 The velum and the foot are retracted into the shell by the action of 

 a pair of long muscles which pass from the sides of the oesophagus 

 and region of the foot to the bottom of the ventral shell, and subse- 

 quently become the columellar muscle of the adult. 



The veliger stage seems to be represented very perfectly in most of 

 the marine Gasteropods, except some of those whose eggs are pro- 

 tected by strong cases, within which the early stages of development 

 are passed. In some of these, as Purpura, there is a well marked but 

 somewhat rudimentary veliger stage, and it is probably represented 

 more or less faintly in all, although the embryo does not pass this pe- 

 riod in free locomotive life, and accordingly has no need of swimming 

 organs. 



Although the marine Opisthobranchs pass through a perfect veliger 

 stage, and are locomotive at this period, the fresh water Pulmonates 

 undergo their embryonic development within the egg, and with them 

 the velum is only faintly indicated, and it appears to be entirely 

 wanting in the land Pulmonates whose young are not aquatic. 



As regards the remaining classes of the Mollusca; the Scaphopods 

 pass through an embryonic form which is easily recognized as a veli- 

 ger, although it is not very highly developed. It would seem as if 

 the Lamellibranchs, from their fixed or nearly fixed mode of life, had 

 an especial need for a locomotive larval stage, but the veliger stage 

 can hardly be detected in this group. Embryos of several of the 

 marine Lamellibranchs have been described and figured as furnished 

 with a circlet of cilia, and thus fitted for locomotion, but these em- 

 bryos are so rudimentary in other respects, and so different from the 

 highly specialized veligers of the Gasteropoda, that we cannot, with 

 any safety, say that they represent this stage of development at all, 



