1 32 GASTROPODA. 



unknown). Each of the velar lobes is drawn out longitudinally so 

 that the whole velum appears to consist of four lobes. In Atlanta, 

 the velum is very large and here each of the lateral parts splits up 

 into three, the velum thus consisting of six lobes (Fig. 67, p. 155). 



In other respects the development of the body has advanced consider- 

 ably at the Veliger stage. The shell, which at first is cup- or cap-shaped, 

 increases in size through the addition of new layers, this fact being 

 indicated, as in Dentalium and the Lamellibranchs, by the appearance 

 of lines of growth. But as the addition of new material takes place 

 in an irregular manner, i.e., as the new layers of shell are not all of 

 equal width and, further, follow the curvature of the visceral sac, the 

 shell soon loses its symmetrical shape and begins to coil (Fig. 55). 

 The visceral sac is separated by the projecting lip-like edge of the 

 mantle from the rest of the body, especially from the head and trunk. 

 A slit-like depression usually appears on the right side in front of the 

 edge of the mantle ; this depression extends posteriorly so that the 

 mantle now covers a cavity, the mantle- (or pallial) cavity, in which 

 the gills arise later as outgrowths of the body-wall. The intestine 

 opens into this cavity, the anus having arisen as an ectodermal 

 depression primarily situated somewhat ventrally at the posterior end 

 of the body. At first this lies in the median plane, but is usually 

 displaced to the right side later, shifting at the same time forward, 

 and somewhat dorsally. This displacement is a result of the fixed 

 and rigid nature of the shell covering a large part of the body (cf. 



p. US). 



The rudiment of the foot appears early and may attain large pro- 

 portions in the Veliger larva. In Vermetus, it is paired at least 

 anteriorly (Fig. 55 B and C). In this larva the foot, however, is 

 more complicated than Fig. 55 would lead us to believe. The double 

 character of its rudiment is noteworthy as a peculiarity which recurs 

 in other Gastropods (Patella, Fig. 51 A ; Limnaea, Bay Lankester, 

 No 63 ; Sucdnea, F. Schmidt, No. 109) in very early stages. In 

 Succinea, the foot arises in the form of two distinct prominences 

 separated by a broad furrow ; these outgrowths afterwards approach 

 each other and fuse to form the median foot, a process similar to that 

 described for Patella (p. 126). 



On the postero-dorsal surface of the foot, a plate composed of the 



same substance as the shell is secreted (Fig. 55 0, op). This is the 



operculum. The otocysts lie in close contact with the foot (B, at). 



In the young stages of the Veliger larva two prominences appear 



on the velar area ; these soon extend and lengthen and can be recognised 



