I3 4 THE GASTROPODA 



The velum is the locomotory ciliated ring, Avhich arises antero- 

 dorsally (Fig. 110, C, w) 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. Ill, 

 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 Atlanta 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, flagellnm in the apical area ; II, 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. I,/), 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 



