258 SNAILS. PHYLUM MOLLUSCA 



digestion as well as most of the absorption takes place, but the 

 secretions are carried forward to the crop, where digestion 

 presumably begins. The finer particles are ingested by the cells 

 of the diverticula, and most, if not all, of the digestion of proteins 

 takes place intracellularly. The snail is one of the few animals to 

 possess an enzyme which can break down cellulose ; what is more 

 remarkable is that although it is entirely vegetarian it can also 

 digest chitin. 



THE KIDNEY AND CIRCULATION 



The ureter, which runs just above the rectum, leads up to a 

 greyish kidney, which produces a variety of nitrogenous products. 

 Its cavity has a minute opening, the renopericardial canal, into 

 the pericardium ; the kidney is in fact a coelomoduct (p. 189). 

 The kidneys should be paired, but the torsion which gastropods 

 undergo has caused the loss of that on the left side. 



The pericardium, which, with the renal cavity, is all that is 

 left of the coelom, lies against the kidney. In it is the heart, con- 

 sisting of an auricle and a ventricle, which drives blood into 

 an aorta. This divides into an anterior branch to the head and 

 foot, and a posterior to the visceral hump. The finer branches 

 of these arteries open into a system of sinuses, constituting a 

 hsmocoele ; this is well seen in dissection, when the dorsal part 

 of the foot is opened, as a large cavity containing the crop and 

 reproductive organs. From the hsemocoele the blood goes to a 

 system of vessels on the roof of the mantle cavity, and so by a 

 pulmonary vein to the auricle. Like the left kidney, the left 

 auricle has been lost. The plasma of the blood contains hsemo- 

 cyanin, a copper-containing protein which, like the haemoglobin 

 of vertebrates (p. 524), assists in the transport of oxygen. The 

 floor of the pulmonary cavity is rhythmically raised and lowered 

 by muscles, so that air is drawn in and out. 



NERVOUS SYSTEM 



All the larger nerves and ganglia are concentrated in the head 

 into a nerve collar which surrounds the gullet. There is a pair 

 of dorsal cerebral ganglia, from which two pairs of nerves run 

 round the gut to a ventral suboesophageal ganglion ; this repre- 

 sents three pairs of ganglia — pedal, pleural, and visceral — which 



