372 PHYLUM MOLLUSC A. 



network of vessels. There the blood is purified. Most of 

 it returns directly to the auricle by a large pulmonary vein, 

 but some passes first through the kidney. 



Eespiratory system. Most Gasteropods, e.g. the dog- 

 whelk (Purpura), the buckie (Bucdnum\ the periwinkle 

 (Littorina), breathe by gills covered by the mantle. The 

 snail being entirely terrestrial, has a pulmonary or lung 

 cavity, formed by the mantle fold. On the roof of this 

 cavity the blood vessels are spread out. Air passes into and 

 out of the pulmonary chamber by the respiratory aperture. 

 When the animal is retracted within its shell, the freshening 

 of the air in the pulmonary chamber takes place by slow 

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

 the chamber is rapidly filled with air, and it is even more 

 rapidly emptied when the body is withdrawn into the shell. 



Excretory system.- -There is a single triangular greyish 

 kidney behind the pulmonary chamber, between the heart 

 and the rectum. It is a sac with plaited walls, and 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. There are two sources of 

 blood supply to the kidney (a) from the pulmonary 

 chamber, and (>} from the heart by a renal artery. As in 

 most other Molluscs, the kidney communicates by a small 

 aperture with that part of the coelom which forms the 

 pericardial sac. Thus, as in earthworm, lobworm, etc., the 

 coelom has a nephridial connection with the exterior. 



Reproductive system.- -The snail is hermaphrodite, and 

 its reproductive organs exhibit much division of labour. 



(a) The essential reproductive organ (the ovotestis} is 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 is probably 

 very rare. 



(b) A much convoluted hermaphrodite duct of a white 

 colour conducts the sex cells from the ovotestis, 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 pro- 

 teid material, which envelops and probably nourishes the ova. 



