358 MOLLUSC A. 



As Tectibranchia must also be included, the 

 Pteropoda, the Winged Snails or Sea Butterflies, 

 which have become modified for pelagic life. They 

 have a secondarily acquired apparent symmetry, 

 and swim by two large lateral lobes of the foot 

 (" parapodia"). They often swim actively in 

 shoals, and occur in all seas. They afford food for 

 whales, &c. , and the shells of some are abundant in 

 the ooze. They include : 



(a) Thecosomata, with mantle fold and shell, diet of 



minute animal or vegetable organisms, closely 

 related to Bulla and its allies. 

 Examples Hyalea, Cymbulia. 



(b) Gymnosomata, without mantle fold or shell in the 



adult. Closely allied to Aplysia and its allies. 

 Actively carnivorous. 

 Examples Clio, Pneumoderma. 



(2) Pulmonata. Air breathing, without gill, the edge of the 

 mantle has fused with the body w^all, forming a 

 pulmonary chamber with a small aperture, e.g., Helix 

 (snail) ; Limax (grey slug) ; Arion (black slug) ; 

 Limnceus, Planorbis, and Ancylus (common fresh water 

 snails). 



GENERAL NOTES ON GASTEROPODS. 



From a form in essentials similar to Chiton, except for its eight shells,. 

 we may consider that the Gasteropods proper have been developed. 

 They are all more or less asymmetrical, but we must notice (l) that 

 this want of symmetry does not affect the head or the foot, but only the 

 dorsal viscera, which are more or less twisted round to the right side 

 towards the head ; (2) the torsion must be distinguished from the 

 frequent spiral twisting of the visceral hump and of the shell ; (3) the 

 torsion occurs in variable degree, and some forms, especially free 

 swimmers, have a superficial symmetry. 



The current explanation of the asymmetry, which has been recently 

 elaborated by Lang, is as follows : 



If we begin with a form something like a Chiton, but with a simple 

 shell, we must suppose the head and foot to become increasingly 

 specialised, and at the same time to acquire an increasing freedom of 

 movement ; during the process the viscera will tend to become more 

 and more limited to a special region of the body, and a "visceral 

 hump " will thus be formed. The shell becomes limited to this region, 

 but the contractility of head and foot, which enables these to be drawn 

 into the shell, must be correlated with the increasing size and complexity 

 of this structure. As, however, shell and visceral hump become larger, 

 they become too heavy to be carried in the primitive position on the 

 back of the animal, and incline to one side. There is, therefore, a one- 

 sided pressure, which results in an increased growth relatively of the 

 opposite side, and so in a deep seated twisting, which brings the 

 originally posterior anus to an anterior position near the mouth, and 

 produces a tendency to the suppression of one of the originally paired 



