GASTEROPODA 481 



The last division of the Gasteropoda is the Pulmonata, which is 

 usually united with the Opisthobranchiata to form the group Euthy- 

 neura. But " euthyneury " or symmetry of the nervous system (more 

 particularly the "visceral" part of it) is arrived at in different ways 

 in the two divisions. In the Opisthobranchiata, as shown above, it is 

 by detorsion. In the Pulmonata, however, the shell is retained and 

 the visceral hump coiled in typical members of the group (land snails). 

 But the visceral loop is shortened and untwisted at the same time 

 (Fig. 363 A, B), and finally it is incorporated with its ganglia into the 

 circumoesophageal nerve collar, so that the nervous system becomes 

 symmetrical. The most primitive members of the Pulmonata still show 

 a twisted visceral loop which is beginning to shorten. All the group 

 have lost the ctenidium but they retain the single auricle which shows 

 them to be derived from the Monotocardia. This was brought about 

 by a chain of circumstances involving migration from sea to shore. 



The type of the Gasteropoda which is usually given for dissection 

 is Helix (either H. aspersa, the common English garden snail, or 

 H. pomatia, the edible snail). It possesses many features which are 

 common to the whole of the Gasteropoda, but as has been seen above, 

 the order Pulmonata to which Helix belongs is the most specialized 

 and probably the latest developed division. Helix is a terrestrial animal 

 breathing by a kind of lung, while the majority of gasteropods are 

 marine animals breathing by gills, and besides the complications 

 which this involves, the reproductive system is hermaphrodite with 

 the most elaborate provision of glands and ducts which serve to 

 produce eggs well stored with nourishment and are arranged so as 

 to assure cross-fertilization. In the account of Helix which follows 

 an attempt is made to distinguish clearly between the purely gasteropod 

 features and the adaptive features which belong to the Pulmonata. 



The body of a snail is composed of three regions, the head, foot and 

 visceral hump. The visceral hump is all that part which is covered by 

 the shell when the animal is expanded, while the head and the foot 

 make up the remainder outside the shell. There is no boundary be- 

 tween the latter two regions. The German zoologists refer to the 

 whole as the " Kopffuss" (the " head foot"), and this can be retracted 

 as a whole within the shell by the action of the columella muscle 

 (Fig. 357). The foot is particularly characteristic of the Gasteropoda. 

 It possesses a flat ventral surface underlain by longitudinal muscle 

 fibres. If a snail is observed crawling up a pane of glass, a series of 

 rippling waves of contraction of very small amplitude are seen to pass 

 regularly over the surface of the foot. They are co-ordinated by the 

 action of a nervous network, such as occurs in the lower invertebrates 

 (Fig. 141). The gliding movement of a snail indeed resembles that of 

 a turbellarian, and we actually find that in some marine gasteropods 

 Bi 31 



