88 COMPARATIVE ANATOMY 



increases greatly in size, acquires a flat, sole-like surface, and_ is 

 used as an organ of locomotion. The velum is reduced in size 

 relatively to the remainder of the body ; a pair of cephahc 

 tentacles grows out of its centre, the velar ring of ciUa atrophies 

 and disappears, and the velar region is converted into the 

 prostomial or head region of the adult. The thickened edge 

 of the mantle secretes a permanent shell which grows out all 

 around the border of the larval shell, and increasing rapidly in 

 size, soon replaces the latter, but for some time the larval shell 

 may be distinguished as a small appendage at the apex of the 

 permanent shell. It is during this further period of growth 

 that the shell and the visceral mass contained in it assume a 

 spiral form. The larval shell is symmetrical, and before the 

 torsion it is exogastric — i.e. its mouth is below and its apex or 

 coiled end is on the anterior face of the visceral mass. After 

 the torsion it is endogastric — i.e. its mouth is above and its 

 coiled end on the posterior face of the visceral mass, but it is 

 still symmetrical, and its shape has not been in any way affected 

 by the torsion. It is clear, then, that the spiral coil of the adult 

 shell is not directly attributable to the torsion of the body. 

 But a glance at fig. 20, Z, shows that when the animal begins to 

 creep on its foot the relatively large and unwieldy shell must 

 fall over to one side or other of the foot. It falls over to the 

 right side, or, rather, the foot places itself obliquely to the 

 aperture of the shell, so that the latter lies on its right side, 

 and in consequence of this asymmetry the permanent shell in 

 course of growth becomes spirally coiled. 



In the snail and other euthyneurous gastropods there is no 

 larval torsion, and consequently no twisting of the visceral 

 nerves. None the less, the anus opens forwards on the right 

 side of the body, and the visceral mass has undergone a cor- 

 responding twist, without involving the nerve centres. The 

 manner in which this twisting of the visceral hump and spiral 

 coihng of the shell are brought about may be gathered from an 

 inspection of fig. 20. 



The embryonic snail has no ciliated velar ring, but the region 

 corresponding to the velum is enormously developed and dis- 

 tended with reserve food material. The shell-gland is, there- 

 fore, inconspicuous, and the development of the shell is retarded. 

 At a later stage, when the nutrient organ is absorbed, the shell 

 is formed, increases rapidly in size, and pushes forward the 



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