352 MOLLUSCA. 



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 larvse (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 respira- 

 tion. 



The precise structure and attachment of the gill-plates is complex, 

 but it is important to understand the following facts : — {a) A cross 

 section of the two gill-plates on one side has the form of a W, one half 

 of which is the outer, the other the inner gill-plate ; (6) each of these 

 gill-plates consists of a united series of gill filaments, which descend 

 from the centre of the W and then bend up again ; (c) adjacent fila- 

 ments are bound together by fusions and bridges both horizontal and 

 vertical, so that each gill-plate becomes like a complex piece of basket 

 work ; (d) both gill-plates begin by the downward growth of filaments 

 from a longitudinal "ctenidial axis," the position of which on cross- 

 section is at the median apex of the W ; (e) this mode of origin, and the 

 much less complex gills of other bivalves, lead one to believe that there 

 is on each side one gill, consisting of two gill-plates formed from a series 



