244 ZOOLOGY. 
nation of the outer germ-layer ; the sides of the primitive 
mouth form the two sails of the velum or swimming organ, 
and the embryo now assumes the veliger stage (Fig. 187). 
Soon the middle germ-layer (mesoderm) arises, and from 
the cells composing it are developed the muscles of the foot 
and head, as well as the heart itself. The mantle or body- 
wall next develops, and from it the shell, which originates in 
a cup-like cavity which is connected only around the edge 
with the mantle, being free in the centre. The eyes and ears, 
or otocysts, next appear, both organs arising as an infolding 
of the outer germ-layer. Hitherto symmetrical, the alimen- 
tary canal now begins to curve to the left, and the visceral 
sac, or posterior part of the embryo hangs over on one side. 
The nervous system is the last to be developed. 
Fig. 188 represents the asymmetrical larva with the shell 
enveloping a large part of the body, and the ciliated velum 
{v) and foot (f) well developed. A temporary larval heart 
(2) assumes quite a different position from the heart of the 
adult, and the primitive, deciduous kidney (/) is situated in 
quite a different place from the permanent kidney. The 
further changes consist in a gradual development of the hel- 
met-like shell, the disappearance of the temporary larval 
structures, and the perfection of the organs of adult life, the 
gills appearing quite late. 
The development of Trochus, the top-shell, exhibits more 
strikingly the trochosphere and 
veliger stages of molluscan life, 
and most Gastropods develop 
like this form. The velum 
at first forms a ciliated ring 
(Fig. 189, A, v) on the front end 
of the trochosphere. Fig. 189, 
B, represents the veliger state. 
It thus appears that the tem- 
: Fig. 189.—Larval Trochus, A, tro- 5 Pte. 
choephere ; 2, velum ; B, veligerstate; POTrary larval or veliger form of 
g, mouth; J, foot; s shell—After the Gastropods are of vermian 
Salensky. 
origin, the organs last to be de- 
veloped, 7. ¢., the foot, shell and lingual ribbon, which are the 
distinctively molluscan characters, being the last to appear. 
