570 THE INVERTEBRATA 



(pneumostome) ; with a single kidney; without a larva, development 

 being direct from an egg richly supplied with albumen. 



The Pulmonata are thus classified : 



Basommatophora. Pulmonata with eyes at the base of the posterior 

 tentacles. Limnaea, Planorbis. 



Stylommatophora. Pulmonata with eyes at the tip of the posterior 

 tentacles. Helix, Arton, Testacella. 



A few members of the Basommatophora are marine but these are 

 all shore forms and breathe air. The group, like the Opisthobranchiata, 

 must have been derived from the Streptoneura Monotocardia, as they 

 possess a single kidney. While they are usually united with the 

 Opisthobranchiata to form the Euthyneura, which includes all forms 

 in which the visceral loop is untwisted, there is no real justification 

 for the establishment of the group, for the " euthyneurous " condition 

 is one which has been arrived at in two different ways, by detorsion 

 in the Opisthobranchiata and by shortening of the visceral com- 

 missures in the Pulmonata. The important characters of the Pul- 

 monata are those associated with the assumption of the terrestrial 

 habit, namely the existence of the lung and the physiological cha- 

 racters correlated therewith. So strongly impressed are these that in 

 almost all the 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. There are, however, a 

 few species {Limnaea abyssalis) which live at great depths in lakes, 

 and here the mantle cavity is full of water. 



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. 390 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 huge proboscis. When the worm is swal- 



