388 PHYLUM MOLLUSCA. 
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. 
Respiratory system.—Most Gasteropods, ¢.g. the dog- 
whelk (Purpura), the buckie (Buccinum), 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rapidly emptied when the body is withdrawn into the shell.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 (4) from the heart by a renal artery. As in 
most other Molluscs, the kidney communicates by a small 
aperture with that part of the ccelom 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. 
(2) The essential reproductive organ (the ovofestis) 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. 
(6) 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 
proteid material, which envelops and probably nourishes 
the ova. 
(2) The ova and spermatozoa pass from the hermaphrodite 
