THE MOLLUSCA 



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. U, 113), Dentalium 



Fio. 14. 



Three Trochospheres of Moll usca. A, /sc/t/ioe/uton, (Heath); B, Patella (Patten); C, Dreisscnsia 

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

 of, 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), 

 which in their turn may be 

 divided into two or three 

 secondary lobes (Fig. 121). 

 (2) The velum may reucttt 

 for a greater or less distance 



towards the posterior end Trochosphere of DrtaZtKi,sagittal median section, 

 nf thp Atnhrvn honnrrnno- w > blastopore ;fl, flagellum ; in, intestine ; /*<, mantle 

 emDryO, DeCOming O r shell-glaml ; w, velum. (Alter Kowalewsky.) 



attached to it in such a 



manner as to form a more or less extensive superficial investment 



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



FIG. 15. 



