496 THE INVERTEBRATA 



and by shortening of the visceral commissures in the Pulmonata. The 

 important characters of the Pulmonata are those associated with the 

 assumption of the terrestrial habit, namely the existence of the lung 

 and the physiological characters correlated therewith. So strongly 

 impressed are these that in forms which have secondarily returned to 

 water (to fresh water as a rule), the lung continues to function as such 

 and never contains water. Limnaea, for example, may be observed 

 in an aquarium to approach the surface of the water at frequent 

 intervals, expel a bubble of air from the lung and protrude the 

 pneumostome through the surface film for a fresh supply. 



The other general characters of a pulmonate have been given at 

 the beginning of the chapter in the description of Helix. They include 

 the concentrated nervous system (it will be seen in Fig. 365 B that 

 the visceral loop of Limnaea is not so much shortened as that of Helix ; 

 in other respects also it is a more primitive form), the complicated re- 

 productive system, with its adaptations for cross-fertilization, and the 

 digestive tract, specialized for the consumption of vegetable food. 

 Helix, as has been seen, is thoroughly adapted for this purpose, but 

 in the case of some of the slugs there is an exception to the general 

 rule in the development of the carnivorous habit. This culminates in 

 such a form as the predaceous Testacella, which pursues earthworms 

 underground and seizes them with the aid of the strong recurved 

 teeth of the radula which can be thrust out of the mouth, the everted 

 buccal cavity forming a kind of proboscis. When the worm is swal- 

 lowed it is digested in a large crop by the action of the juices of the 

 digestive gland. 



The reduction of the shell is shown in the slugs, some of which, 

 like Testacella, have a small cap-like shell, which cannot possibly con- 

 tain the visceral hump, while others have an internal horny disc like 

 the shell of Aplysia and still others none at all. In other respects the 

 organization of the slugs is very similar to that of snails. 



The details of reproduction and development are very similar 

 throughout the group, but in some snails like Bulimus, the amount of 

 albumen added as food for the developing embryo is so great that the 

 egg is the size of a bantam's egg. 



Class SCAPHOPODA 



Bilaterally symmetrical MoUusca with a tubular shell open at both 

 ends, a reduced foot used for burrowing, a head with many pre- 

 hensile processes, a radula, separate cerebral and pleural ganglia; 

 ctenidia absent and circulatory system rudimentary; and a trocho- 

 sphere larva. 



This is a small group of molluscs which in some ways stands be- 



