GASTROPODA. 385 



A glance at the accompanying diagram will explain the effect of 

 the first-described torsion about the dorso-ventral axis upon the 

 organs of the visceral sac, and 

 the course of the peculiar figure- 

 of-8 loop of the visceral com- 

 missure. AVhy the organs of 

 the primitive left side disappear 

 in so many Gastropods is not 

 clear, but their disappearance is 

 doubtless connected in some way 

 with the torsion, and consequent Fm ~^_ Ampullaria cornu arietis (rtgne 



asymmetry produced in the parts animal), illustrating the spiral torsion of 



ff i , ,, . the visceral sac about a right and left 



affected by the torsion. horizontal axis, in which the visceral sac 



111 SOme groups there is a has apparently fallen backwards (endo- 



., . gastric). It is, however, really an exogastric 



tendency lor the Visceral Sac to coili beca use the visceral sac has been 



flatten out into a conical form, and twisted so that its hinder surface is in 



for the nautiloid shell to become 



conical (Patella, Fissurella, Aplysia, Pleurobranchus, Doris, etc.). 

 This is, however, a purely secondary feature, for the young possess 

 the typical nautiloid shell, and in some cases the stages of the loss 

 of the spire can be completely followed. 



This acquisition of a secondary symmetry, Avliich is found .very 

 commonly in the Eutliyneura, must not, however, be confused with 

 a tendency to detorsion of the visceral sac which is found in most 

 Eutliyneura. It shows itself in the shifting back of the anus well 

 on to the right side and even to the hind end, and in the untwisted 

 condition or only partially twisted condition of the visceral loop (see 

 account of Adaeon and allies). Formerly this condition was looked 

 upon as an arrested stage in the torsion, but having regard to the 

 arrangement of the visceral commissure in Adaeon and its allies, and 

 to the fact that in the suppression of the organs of the right 

 (primitive left) side the Eutliyneura are as specialized as the most 

 specialized Streptoneura, it is more probable that the condition 

 referred to is due to detorsion. Forms in which the anus is 

 secondarily shifted backwards almost always present a reduction 

 or disappearance of the mantle and shell (Pterotracliea, Aplysia, 

 Dorids. Janus, Alderia, Limapontia, Testacella, Oncliidium, Vagimdus). 



By far the majority of the Gastropoda are marine. Almost the 

 whole of one group of the Pulmonata (Basommatopliora] and the 

 following Streptoneura live in fresh water some Neritidae, the 

 Ampullariidae, Paludinidae, Valvatidae, Bitliyniidae, Hydrol>iidae, 



2 c 



