RESPIRATORY AND EXCRETORY SYSTEMS. 369 



Respiratory System. 

 Lying between the mantle flaps and the foot there are on 

 each side two large gill plates, whence the title Lamelli- 

 branch. They are richly ciliated, their internal structure is 

 like complex trellis work, their cavities communicate 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 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. 



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 ; {b) 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 

 of united and reflected gill filaments. On the gills there are often 

 parasitic mites (Atax). 



Excretory System. 



The paired kidney, which used to be called the " organ 

 of Bojanus," lies beneath the floor of the pericardium. 

 Each half is a nephridium bent upon itself, with the loop 

 posterior, the two ends anterior. The lower part of this 

 bent tube is the true kidney ; it is dark in colour, spongy in 

 texture, and excretes guanin and other nitrogenous waste 

 from the blood which passes through it. It has an internal 

 opening into the pericardium, which thus communicates 

 indirectly with the exterior. The upper part of the bent 

 tube, lying next the floor of the pericardium, is merely a 



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