SOLENOCONCHA. 



While the nervous system is forming through the processes just 

 described, both the ciliated tuft and the ciliated ring undergo reduc- 

 tion (Fig. 37). This is especially 

 the case with the latter which, in 

 accordance with the nomencla- 

 ture used for other Molluscs,* 

 is here also called the velum. 

 The velum is the chief swim- 

 ming organ and, when it degene- 

 rates, the larva has to adopt 

 another method of locomotion. 

 At the stage depicted in Fig. 

 38, the velum appears still 

 greatly developed, but, as the 

 conical apical pole has degene- 

 rated, the anterior section of 

 the larva now) seems flattened 

 and plate-like. When the 

 velum is more reduced and 

 the other parts of the body 

 (the shell, the foot, etc.) better 

 developed, the larva sinks to 

 the bottom, where it still swims 

 to some extent by means of the 

 velum, but also creeps with 

 the assistance of its foot, just 

 as do other Molluscan larvae 

 when passing over to the adult 

 form (cf. p. 42 and Figs. 53, 

 54, 67, etc.). The free-swim- 

 ming life of the larva lasts 

 FIG. 37. A-C, frontal sections through . 



older larval stages of Dentalium, showing quite four days, during which 

 the formation of the brain (after KOWA- , - j f i:u ^hp 



LEVSKY). eg, rudiment of the cerebral time Ot ' 11RG 



ganglion ; m, mantle ; oes, stomodaeum ; larvae of the Lamellibranchia 

 s. cephalic pole ; w, pre-oral ciliated ring. 



and the Gastropoda, move at 



the surface of the water, but appears to maintain itself at various 



depths (LACAZE-DUTHIERS). 



* Cf. on this point pp. 33 and 125. 



