140 Affinity of MoUusea and MoUuscoida. By W. K. Brooks. 
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 
structure ; in most veligers this circulation seems to be aided by 
the rhythmical contraction of the muscular fibres which bind the 
foot to the oesophagus. The mouth is not within the circlet of 
large locomotive cilia, but immediately behind it, and a ring or band 
of smaller cilia passes from the anterior margin of the mouth en- 
tirely 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 subsequently 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 
protected by strong cases, within which the early stages of deve- 
lopment 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 period 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 Scapho- 
pods pass through an embryonic form which is easily recognized as 
a vehger, 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 furnislied with a circlet of cilia, and thus fitted for loco- 
motion, but these embryos are so rudimentary in other respects, 
and so difierent from the highly specialized veligers of the Gastero- 
poda, that we cannot, with any safety, say that they represent this 
stage of development at all, although the fact that Anodonta has an 
unmistakable velum would seem to indicate that the Lamellibranchs, 
like the Gasteropods, are the descendants of a free-swimming vehger, 
and that the circlet of ciHa described in the embryos of such forms 
as Cardium is also to be regarded as a rudiment of the same stage. 
It may be that the development of the young within the branchiae 
