PHYLUM MOLLUSC A 195 



modification of the respiratory system is found in the pulmonale 

 snails. Here the walls of the mantle cavity become richly 

 supplied with blood vessels and the cavity becomes recognizable 

 as a lung. A small opening permits air to be taken directly into 

 the lung. This is the usual method of respiration in most of the 

 land snails. The aquatic pulmonates seem to be closely related 

 to the land forms, and members of the genera Physa, Planorbis, 

 and Lymnaea have a lung similar to that found in land snails. 



Sense Organs. — The tentacles are the most conspicuous 

 sensory organs. The eyes may be either at the bases of or at 

 the tips of the tentacles but in some forms they are borne at the 

 tips of a second pair of tentacles. 



Torsion of Body. — Modifications of the body correlated with 

 torsion (Fig. 92) have in some instances resulted in loss of gill, 

 nephridium, and osphradium of the primitively left side. As in 

 the other Mollusca, the number of auricles is directly correlated 

 with the number of respiratory organs, consequently with the 

 loss of one gill the heart comprises but a single auricle and a single 

 ventricle. When the lungs or gills are located in the front of 

 the heart (Prosobranchia and Pulmonata), the auricles are 

 anterior to the ventricle, but when placed behind the heart 

 (Opisthobranchia and Pteropoda), the auricle is posterior to 

 the ventricle. 



Reproduction. — The gonad is always single. In some instances, 

 the sexes are separate but many genera are hermaphroditic. 

 In the latter, though there is a single hermaphroditic gonad, each 

 individual bears the accessory sexual organs of both sexes. There 

 is great diversity in reproductive habits. In many species eggs 

 are laid singly or in small groups, in others they are deposited in 

 masses surrounded by a gelatinous substance, while in many of 

 the marine snails complicated capsules are formed for containing 

 the eggs and an albuminous fluid which serves to nourish the 

 embryos. The course of development is fairly uniform through- 

 out the group, involving the presence of a veliger larva. In 

 terrestrial and fresh-water forms, the veliger stage is passed 

 within the eggshell and not as a free larval stage. Some gastro- 

 pods are viviparous. 



Subclass Prosobranchia 



In the Prosobranchia or Streptoneura are included those 

 gastropods which have the cercbrovisceral commissures of 



