68 Elementary Zoology. 



lies below the latter, while behind the tentacles is, on the right 

 side, a pore, connected with an external groove, the generative 

 aperture. Apart from the unilateral generative pore, that 

 portion of the body which lies outside the shell is bilaterally 

 symmetrical. There is no more evidence of segmentation in 

 the snail than there is in the Anodon. The shell of the snail 

 has a spiral cavity; it is formed by the coiling of a shelly 

 tube, the common wall formed by the apposition of the coils 

 being itself at first hollow. The term columella is applied to 

 this. That the shell is univalve implies no marked distinction 

 from the shell of Anodon. It is simply due to the continuous 

 secretion of calcareous matter instead of its secretion in two 

 plates, as in the Anodon. The mantle cavity is a spacious 

 chamber corresponding, of course, to the spaces which lie 

 beneath the mantle-flaps in Anodonta. It is, therefore, morpho- 

 logically a part of the exterior. The name " lung " sometimes 

 applied to this cavity is, therefore, so far misleading. Never- 

 theless, this chamber performs the function of a lung, as the 

 animal has no gills such as are found in other Cephalous 

 Mollusca. The walls of the mantle cavity are thin, and just 

 beneath the integument is an abundant plexus of blood-vessels, 

 whose blood is thus brought into near relations with the air 

 contained in the "lung." This (the air) is renewed and ex- 

 pelled at intervals, as already mentioned. Into the mantle 

 cavity open the anus and the renal gland. 



When the snail is dissected the same absence of ccelom that 

 characterizes Anodonta is to be remarked. As in that animal, 

 the only remaining portion of the ccelom is the pericardium, 

 which surrounds the heart. No other organs, however, lie 

 within the pericardium, but the single renal organ opens into it 

 by a reno-pericardial pore. 



The alimentary canal is more complex than that of Anodon. 

 The mouth leads into a cavity called the buccal mass, which 

 has thick muscular walls, and whose floor is supported by 

 cartilage. Upon this cartilage lies a structure characteristic of 

 all the Cephalous Mollusca known as the radtila. This is a 

 stiff ribbon which is developed into a series of calcified teeth, 

 closely set and overlapping each other. The ribbon bearing 



