452 



PHYLUM MOLLUSCA 



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 — one on each side of the 

 ventricle — which receive blood returning from the gills and 

 mantle. In bivalves the heart-beats average about twenty 

 per minute, far fewer than in Gasteropods. The colour- 

 less 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 peri- 

 cardium ; passes thence through the kidneys, where it 

 loses nitrogenous waste, to the gills, where it loses car- 

 bonic 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 freed from its waste 

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

 of " pericardial glands " on the epithelium of the pericardial 

 cavity. They seem to be connected with excretion. 

 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 large gill-plates, 

 whence the title Lamelhbranch. They are richly ciUated ; 

 their internal structure is like complex treUis-work ; their 

 cavities communicate with the supra-branchial chamber. 

 As in many other Molluscs, the gills or ctenidia are not 

 merely 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 larvae (a reproductive function). 

 The water may pass through the gills to the supra-branchial 

 chamber and thence out again, or over the gills to the 

 mouth, and thence into the supra-branchial chamber. It is 

 likely that the mantle has no small share in the respiration. 

 In many cases, e.g. Lutraria elliptica, the posterior end of 

 the mantle gives origin to a contractile respiratory siphon, a 

 double tube, the upper half of which is expiratory and the 

 lower half inspiratory. A cross-section shows a cuticular 

 investment of conchin, a layer of epidermis, a narrow zone 

 of circular muscle-fibres, a thick zone of longitudinal 



