374 MOLLUSC A. 



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



adult. Closely allied to Aplysia and its allies. 



Actively carnivorous, e.g. Clio, Pneumoderma. 



B. Nudibranchiata. Shell, mantle fold, and true gill are absent ; 



various forms of ' ' adaptive gills " may be present, or there 



may be no special respiratory organs, e.g. sea-slugs, Doris, 



Eolis, Dendronotus (Fig. 160). 



It will be obvious from this table that the classification of the 

 Gasteropods cannot be greatly simplified. The essential points may 

 perhaps be summarised as follows : — In the order Prosobranchia are 

 included, first, primitive forms with more or less simple conical shells 

 and with traces of the primitive bilateral symmetry ; and, second, the 

 greater number of these marine Gasteropods which have well-developed 

 conical shells closed by an operculum, as well as the modified pelagic 

 Heteropods. The Pulmonata are readily recognised, and the Opistho- 

 branchiata include (in general terms) marine Gasteropods usually without 

 conspicuous shells, and often much modified in external appearance, 

 and also the aberrant pelagic Pteropods or sea-butterflies. 



From a form somewhat like a Chiton, but with a simple conical shell, 

 we may consider that the Gasteropods proper have been developed. 

 They are all more or less asymmetrical, but we must notice — (i) 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 

 gills, nephridia, etc. According to Lang, during the torsion an increased 

 growth of the upper surface of the visceral mass is necessary in order to 

 avoid rupture, and thus the superficial spiral coiling is produced ; this is 

 reflected in the coiling of the shell. In one series of the Gasteropods 

 the visceral nerve loop, running from the cerebral and pleural to the 

 visceral ganglia, is "caught in the twist," and twines like a figure 8 



