3o8 MOLLUSCS. 



stomach is compacted in autumn into a " crystalline style "— - 

 a mass of reserve food-stufifs, and similar but less solid 

 material is found in the intestine. On this supply the niussel 

 tides over the winter. Some authorities, however, maintain 

 that the style is a glandular secretion, protecting the lining of 

 the gut from injury. The intestine, which has in part a folded 

 wall like that of the earthworm, coils about in the foot, 

 ascends to the pericardium, is surrounded by the ventricle 

 of the heart, and ends above the posterior adductor at 

 the exhalent orifice. 



Vascular System. — The heart lies in the middle line on 

 the dorsal surface, within a portion of the body-cavity called 

 the pericardium, and consists of a muscular ventricle which 

 has grown round the gut and drives blood to the body, and 

 of two transparent auricles which receive blood returning 

 from the gills and mantle. The colourless blood passes 

 from the ventricle by an anterior and a posterior artery, flows 

 into ill-defined channels, is collected in a " vena cava " 

 beneath the floor of the pericardium, passes thence through 

 the kidneys, where it loses nitrogenous waste, to the gills 

 where it loses carbonic acid and gains oxygen, and returns 

 finally by the auricles to the ventricle. The blood from 

 the mantle, however, returns directly to the, auricles without 

 passing through kidneys or gills, but probably rid of its 

 waste none the less. The so-called " organ of Keber " con- 

 sists of " pericardial glands " on the epithelium of that 

 cavity. They seem to be somehow connected with excre- 

 tion. Many of the cells lining the blood-channels secrete 

 glycogen, the principal product of the Vertebrate liver. 



Respiratory System. — Lying between the mantle flaps and 

 the foot there are on each side two gill-plates, whence the 

 title Lamellibranch. They are richly ciliated, their internal 

 structure is hke complex trellis-work, their cavities communi- 

 cate with the supra-branchial chamber. " Ctenidia " they are 

 often called, because they are more than gills ; not only are 

 they surfaces on which blood is purified by the washing water- 

 currents (a respiratory function), but some of their many cilia 

 waft food-particles to the mouth (a nutritive function), and 

 in the females the outer gill-plate shelters and nourishes the 

 young larvse (a reproductive function). The water-current is 

 twofold — (i) through the gills to the supra-branchial chamber 



