264 



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. 192). 

 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. 193 represents the asymmetrical larva with the shell 

 enveloping a large part of the body, and the ciliated velum 

 (v) and foot (/) well developed. A temporary larval heart 

 (h) 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 



,., ,, . X ml 



like this form. The velum 

 at first forms a ciliated ring 

 (Fig. 194, A, v) on the front end 

 of the trochosphere. Fig. 194, 

 B, represents the veliger state. 



It thus appears that the tem- 

 porary larval or veliger form of 

 '' she1l - After the Gastropods are of vermian 

 origin, the organs last to be de- 

 veloped, i. e., the foot, shell and lingual ribbon, which are the 

 distinctively molluscan characters, being the last to appear. 



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