STRUCTURE OF THE SNAIL. 325 



(Littorind), breathe by gills covered over by a fold of 

 the mantle. The snail being entirely terrestrial has a pul- 

 monary or lung cavity, formed by the usual mantle flap. 

 On the roof of this cavity the blood-vessels are spread out, 

 and the blood is purified. Air passes into and out of the 

 pulmonary chamber by the respiratory aperture, which lies, 

 as we have mentioned, on the right side of the body. When 

 the animal is retracted within its shell, the freshening of 

 the air in the pulmonary chamber must take place by slow 

 diffusion, but when the snail extends itself at full length, the 

 chamber must be rapidly filled with air, and even more rapidly 

 emptied when the animal withdraws into the shell again. 



The Excretory System consists of a triangular greyish 

 kidney, which lies behind the pulmonary chamber between 

 the heart and the rectum. It is a sac, with plaited walls, 

 and certainly excretes nitrogenous waste-products, which 

 pass out by a long ureter running along the right side of the 

 pulmonary chamber, and opening close beside the anus. 

 From two sources the kidney is supplied with pure blood, 

 from the pulmonary chamber, and from the heart by a renal 

 artery. As in other Molluscs, the kidney communicates by 

 a small aperture with that part of the body-cavity which 

 surrounds the heart and is known as the pericardial sac. 

 This pericardium has therefore an indirect communication 

 with the exterior. 



The Reproductive System. — The snail is hermaphrodite, and 

 its reproductive organs exhibit much division of labour. 



(a) The essential reproductive organ (or ovo-testis) is 

 readily seen as a whitish body near the apex of the visceral 

 spire. It consists of numerous cylindrical follicles, in each 

 of which both ova and spermatozoa are formed, but not at 

 the same time. Simultaneous formation of elements so 

 different would be a physiological impossibility. 



{b) A much convoluted readily broken hermaphrodite duct 

 of a white colour conducts the germ-cells from the ovo- 

 testis, and leads to the base of a large yellowish albumen 

 gland. 



{c) This tongue-shaped albumen gland varies in size with 

 the age and sexual state of the snail. It forms gelatinous 

 proteid material, which appears to envelope and perhaps 

 nourish the ova. 



