40 RESPIRATION. 



Many univalves have such an extraordinarily large foot ( Cymba 

 neptunis, Buccinum laevissimum, Harpa, Xatica. Sigaretus, etc.) 

 that it often exceeds the volume of the shell many times when 

 fully expanded ; and it has long been cause for conjecture how 

 this mass was retracted into the shell. By the discovery of the 

 water-vascular system and its connection with the body cavity 

 the mode in which the foot is retracted becomes clear, since the 

 foot is filled with water in the same way MS :t sponge, which 

 escapes again when it is retracted so that the increase of the 

 mass of the foot is due to the water it has sucked up. Delle 

 Chiaje already understood the relation of the water-vascular 

 system to this process, though upon the whole he ascribed a 

 respiratory function to the water which entered the body cavity. 

 Osier also attributes the enlargement of the foot in Buccinum 

 to the water absorbed by the water- vascular system, well known 

 to him. Agassiz introduced quite decisive experiments upon 

 this point. He placed a large Natica heros with the foot re- 

 tracted, in a glass vessel of water full to the level of the brim, 

 and when the animal gradually produced its enormous foot not 

 a drop of water was spilled from the vessel. He performed 

 similar experiments upon many univalves and lamellibranchs in 

 graduated glass tubes and in all movements and variations in 

 the size of the foot there never was the slightest difference to 

 be detected in the water-level. 



Respiration. 



The branchiae in prosobranehiates are small, leaf-like, hollow 

 prolongations of the mantle, placed in rows behind each other, 

 and are usually contained in a pouch on the dorsal side of the 

 animal, forming the respiratory cavity. In some opisthobran- 

 chiates the gills are lodged at the sides of the body between the 

 narrow, collar-like mantle and the broad foot : such is the posi- 

 tion of the filamentous branchiae of Patella and the laminated 

 gills of Chiton. In the spiral species the right branchia only is 

 well developed, that on the left side being small and rudimentary; 

 sometimes, however, as in Turbo and Phasianella, the two gills 



last open directly into the previsceral cavity, or whether they are dis- 

 tributed only in the foot.Sitzungsb. Akad. Wiss. Wien, ii, 1868. Ann. 

 Mag. Nat. Hist., 4 ser., IV, 365. 



