THE MOLLUSCA 27 
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), Dentalium 
Fig. 14. 
Three Trochospheres of Mollusea. 4, [schnochiton (Heath); B, Patella (Patten) ; C, Dreissensia 
(Meisenheiiner). a, anus; f, foot; fl, flagellum; m, mouth; p.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), 
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 Fic. 15. 
towards the posterior end — trochosphere of Dentativm,sagittal median section. 
of the embryo, becoming ¥%Qsstopore:#,fayetium; i intestine: yo, mantle 
attached to it in such a 
manner as to form a more or less extensive superficial investment 
furnished with multiple ciliated rings, as in Dentaliwm (Figs. 15, 
