ANATOMY AND PHYSIOLOGY OF A SNAIL. 35 



monary vessels, the pulmonary vein, and the circulus venosus 

 pulmonis. External respiration can be readily performed then 

 by this vascular network. In the branchiate Gastropoda respira- 

 tion is performed by a ctenidium or gill, the water containing air 

 being admitted in the majority of cases by an anterior siphon^ and 

 expelled by a posterior siphon. These gills can be well seen in 

 Vivipara vivipara. Cydostoma elegans and Aricula lineata are 

 examples of a branchiate becoming a pulmonate ; they have lost 

 their ctenidium, and respiration is now carried on by the walls of 

 the pulmonary sac. These are the only two English representatives 

 of the PNEUMONOCHLAMYDA, a class which has been proposed to 

 contain all those terrestrial Mollusca in which the ctenidium aborts, 

 and respiration is afterwards carried on by means of a lung. Vivi- 

 para vivipara has a ctenidium, and consequently it belongs to an 

 allied class termed the HOLOCHLAMYDA in which the ctenidium is 

 present in the adult. The passage from the branchiate to the pul- 

 monate form is well seen in the genus Ampullaria, where a ctenidium 

 is to be found on the left side of the mantle-cavity, while the right 

 side, which is separated from the left by a fold, is extremely vascular 

 in character. The floor of the mantle is formed by a sheet of muscle 

 tissue. Inspiration is effected by a contraction of this muscular 

 layer, which bulges up when at rest into the mantle cavity just as 

 our diaphragm does in our chest cavity ; expiration is effected by 

 the return of the muscular layer to a state of relaxation. 



A somewhat similar condition of things present in Ampullaria 

 has been found by Semper, it is interesting to note, in a terrestrial 

 Crustacean, Birgus latro. According to Sabatier, the blood is 

 driven, in Ampullaria, to the gills when the lung is collapsed, and 

 to the lung when the gills are collapsed, by means of a valvular 

 spur situated at the junction of the afferent vessel of the gills, and 

 the afferent vessel of the lung. Some years back a good deal of 

 discussion was taking place in the Continental magazines about 

 the respiration of the Ltmnceida in deep water, and during winter 

 when the ponds are covered with ice, the outcome of which was the 

 interesting fact that under such circumstances they do not respire 

 atmospheric air direct, but admit water to their pulmonary cavity 

 instead. 



THE NEPHRIDIUM. The kidney or nephridium is situated in 



