134 THE GASTROPODA 
The velum is the locomotory ciliated ring, which arises antero- 
dorsally (Fig. 110, C, vr) and circumscribes the apical area. As in 
the Amphineura, it is only slightly prominent in the most archaic 
marine Aspidobranchia (Patella, Fig. 14, Acmaea, Trochus, Fig. 111, 
ve), but in other Gastropods it becomes more and more prominent, 
and eventually gives rise to a natatory velum formed of two 
lateral lobes with ciliated margins (Fig. 112). These lobes may 
in turn be subdivided into two (Fig. 121, ve) or three secondary 
lobes (for example, in Aflanta and in “ Ethella,” a larval form 
attributed to one of the Strombidae). In viviparous species, or in 
forms like Cenia and the Pulmonata, in which the young’ are 
Fig. 113. 
Trochosphere of Patella, in the 34th hour. I, flagellum in the apical area; IJ, left lip of the 
blastopore ; III, blastopore; IV, velum. (After Patten.) 
hatched in the adult condition, the velum becomes rudimentary or 
disappears altogether; in the Basommatophora (Fig. 119, A) it is 
developed to a slight degree, but only on the sides, and is not con- 
tinuous ; in the Auriculidae, Siphonariidae, and Oncidiidae, however, 
all of which are marine, a normal velum is present. 
The foot is always very short at first (Fig. 1, f), being repre- 
sented by a mere papilla, which in sundry instances is formed from 
two paired rudiments (Fig. 113, II) situated between the two ex- 
tremities of the primitive blastopore when this latter structure 
is elongated. The operculum is formed early, and exists in all 
testaceous larvae, even in cases in which the adult possesses neither 
shell nor foot: the only exceptions to this statement are the 
Pulmonata (excepting the Auriculidae, the Amphibolidae, and the 
