GASTROPODA. 



385 



FIG. 303. Ampullaria cornu arietis (regne 

 animal), illustrating the spiral torsion of 

 the visceral sac about a right and left 

 horizontal axis, in which the visceral sac 

 has apparently fallen backwards (endo- 

 gastric). It is, however, really an exogastric 

 coil, because the visceral sac has been 

 twisted so that its hinder surface is in 

 front. 



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. Why 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 

 asymmetry produced in the parts 

 affected by the torsion. 



In some groups there is a 

 tendency for the visceral sac to 

 flatten out into a conical form, and 

 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, which is found very 

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

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

 Euthyneura. 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 Actaeon and its allies, and 

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

 (primitive left) side the Euthyneura 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 (Pterotrachea, Aplysia, 

 Dorids, Janus, Alderia, Limapontia, Testacella, Onchidium, Vaginulus). 



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

 whole of one group of the Pulmonata (Basommatophora) and the 

 following Streptoneura live in fresh water some Neritidae, the 

 Ampullariidae, Paludinidae, Valvatidae, Bithyniidae, Hydrobiidae, 



2 c 



