ANATOMY AND PHYSIOLOGY OF A SNAIL. 35 



mornry 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 

 Paludina vivipara. Cydostoma elegans is an example of a bran- 

 chiate becoming a pulmonate ; it has lost its ctenidium, and 

 respiration is now carried on by the walls of the pulmonary sac. 

 It is the only English example of the PNEUMOCHLAMYDA, a class 

 proposed by Professor Ray Lankester to contain all those terres- 

 trial Mollusca in which the ctenidium has aborted, and respiration 

 carried on by means of a lung. Paludina vivipara has a ctenidium, 

 and consequently it belongs to an allied class termed the HOLO- 

 CHLAMYDA in which the ctenidium is present in the adult. The 

 passage from the branchiate to the pulmonate 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. Inspira- 

 tion is effected by a contraction of this muscular layer, which 

 bulges up when at rest into the mantle cavity just as our dia- 

 phragm 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, Bi,rgus 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. A few years back a good deal of 

 discussion was taking place in the Continental magazines about 

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

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

 interesting fact that under such circumstances they do not respire 

 atmospheric air direct, but admit water to their pulmunary cavity 

 instead. 



THE NEPHRIDIUM. The kidney or nephridium is situated in 



