100 COMPARATIVE ANATOMY CHAP. 



displacement of the visceral dome and mantle to the posterior end of 

 the body, will be found in Section V., p. 76. 



Certain Pulmonata (Limnceidce) have become readapted to aquatic life, but their 

 respiration is the same as that of the terrestrial forms, they rise periodically to the 

 surface of the water to take in air. The respiratory cavity, is, however, tilled with 

 water when the animal is young, and it is then a water breather. In Limncea 

 abyssicola, a deep-water form found in the lake of Geneva, this form of aquatic 



cr 



FIG. 95. Helix. The roof of the pulmonary sac cut along the rectum, and along the edge 

 uniting with the nuchal integument, and turned back to show the arrangement of the blood 

 vascular system, after Howes. The pulmonary veins are of a lighter shade than the afferent 

 pulmonary vessels and the venous sinuses ; , bl>, show the cut edges which belong to each other ; 

 1, afferent pulmonary vessels which draw their venous blood from the large circular venous 

 sinus (9); this latter receives its blood from the large sinuses of the body, two of which, 

 that of the visceral dome (6) and that on the right side of the foot (7) are shown. The efferent 

 pulmonary vessels collect the blood which has become arterial on the roof of the pulmonary 

 chamber, and conduct it through the pulmonary vein (2) to the auricle (3) ; 4, ventricle ; 5, renal 

 circulatory system. 



respiration continues throughout life, and the pulmonary chamber, in no way 

 modified, is constantly filled with water. 



In certain terrestrial Prosobrandiia (Cydostoma, Cydophorus, etc.) the 

 respiratory cavity becomes transformed, as in the Pulmonata, into a 

 pulmonary chamber, and its roof is covered with a respiratory 

 vascular network. But there is here no concrescence of the edge of 

 the mantle with the nuchal integument. Cydostoma still retains a 

 rudiment of a prosobranchiate gill, but this is lost in Cydophoi'us. 

 The amphibian Ampullaria possess both a gill and a pulmonary 

 sac, 1 and can breathe either water or air. 



1 See note ante, p. 90. 



