THE MOLLUSC A 



adaptation to larval life. The most important of these embryonic 

 organs is the velum; primitively a preoral ciliated ring, characteristic 

 of the Trochosphere (Ray Lankester), which delimits an area known 

 as the prostomium or "velar area." Towards the centre of this 

 area there is often a ciliated tuft or a flagellum, as in the larvae 

 of Amphineura (Figs. 14, 17), Patella (Figs. 14, 113), Ikntnlium 



tl 



FIG. 14. 



Three Trochospheres of Mollusca. A, Ischnochiton (Heath); B, Patella (Patten) ; C, DK 

 (Mcisenheimer). , anus ; /; foot ; fl, flagellum ; m, mouth ; j>.a.c, post-anal cilia ; sh, shell ; 

 ve, velum. 



(Fig. 15), and various Lamellibranchs (Figs. 14, C; 16). It is on 

 the buccal side of this tuft, when it exists, that the apical plate 

 is .situated, probably the remains of a sensory organ from which the 

 cerebral nerve-centres take their origin. The velum may be differ- 

 entiated in one of two ways: (1) The preoral ciliated ring may 

 extend itself by growing outwards at all parts of its circumference : 

 the trochosphere larva is thus 

 transformed into a " veliger " 

 (Ray Lankester), a larval 

 form highly characteristic of 

 Molluscs (Fig. 18). The 

 velum may be divided into 



two lateral lobes (Fig. 18, v), \ - 1\*\\ -\." ^- ^* 



which in their turn may be 

 divided into two or three 

 secondary lobes (Fig. 121). 

 (2) The velum may retreat 

 for a greater or less distance 

 towards the posterior end 

 of the embryo, becoming 

 attached to it in such a 

 manner as to form a more or less extensive superficial investment 

 furnished Avith multiple ciliated rings, as in Dentalium (Figs. 15, 



U 



Fio. 15. 



Trochospliereof />/>/<'/( /. sagittal median section. 

 lil, blastopore ; fl, flagellum ; in, intestine ; pu, mantle 

 or shell-gland ; ve, velum. (After Kowalewsky.) 



