398 PHYLUM MOLLUSC A. 



through the ventricle of the heart, and ends above the 

 posterior adductor at the exhalant 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 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, much less 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 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 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 Lamellibranch. They are richly ciliated ; 

 their internal structure is like complex trellis-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 



