DEVELOPMENT OF THE EXTERNAL FORM PTEROPODA. 



171 



is connected with the mantle or s 

 the left dorsal side. 



hell (in the Ca&olimidae) only on 



The ventral position of the mantle-cavity in the Cavoliniidae is very striking, 

 as this cavity, in other Gastropods, is dorsal in position. According to BOAS, 

 the visceral dome and the shell connected with it have undergone torsion. 

 This view is supported by the fact that, in younger larvae, the bent apex of 

 the shell is directed not, as in the adult, dorsally, but to the left. It is at 

 once evident that this process may be classed with those already described in 

 connection with the acquisition of asymmetry by the Gastropoda (p. 143), but 

 in this case other changes have been added in adaptation to a different manner 

 of life. 



We shall not here give any special account of those ontogenetic 

 processes such as the formation of the otocysts, the radular sac, etc., 

 which take place in the same way as in other Gastropods. 



Gymnosomata. The Trochophon is followed by a larva provided 

 with a large bilobed velum 

 and a pointed foot (Fig. 

 7T> A,/). The shell, which 

 at first is cup-shaped but 

 later oviform, as it grows in 

 length, becomes a tube 

 widening out anteriorly 

 (Fig. 76 A), on which, as a 

 rule, the zones of growth 

 are recognisable as intervals 

 varying in width. The 

 larva does not long remain 

 at this stage, in which it 

 closely resembles the 

 straight -shelled Thecoso- 

 mata. The shell is thrown 

 off and the velum degene- 

 rates. When it disappears, 

 01- even sooner, three ciliated 

 rings appear on the body 

 (Kiirs. 76 A' and 774). In 

 those larvae that develop 

 ciliated rings even before 

 the disappearance of the 

 velum and the casting of 

 tlu shell, these are distri- 

 buted in such a way that the most anterior ring lies between the 



Fi(i. 76.- Larvae of Clio tie at two different stages 

 >HN and G E< j E N BA UK ) . 

 ach ; ne, oesophagus ; 

 x, shell ; v, velum ; w, 



of development (after KROHN and GEOENBAUR). 

 /, foot ; /, liver ; //t, stomach ; oe, oesophagus ; 



retractor muscle ; 

 ciliated rings. 



