STRUCTURE OF ANODONTA. 309 



and thence out again, (2) over the gills to the mouth, and thence 

 into the supra-branchial chamber or in part into the food- 

 canal. It is likely that the mantle has no small share in the 

 respiration. 



I shall not' describe the precise structure and attachment of the gill- 

 plates, 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 ; i]i) 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) considering this mode of 

 origin, and the much less complex gills of other bivalves, it is believed 

 that it is most accurate to say that there is on each side one gill, con- 

 sisting of two gill-plates formed from a series of united and looped gill- 

 filaments. 



The Excretory System consists of a paired kidney which used 

 to be called the " organ of Bojanus." The two kidneys lie 

 side by side beneath the floor of the pericardium. Each 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, i.e., into part of the body-cavity. The 

 upper part of the bent tube, lying next the floor of the peri- 

 cardium, is merely a canal analogous with a ureter. It 

 conveys waste-products from the glandular part to the 

 exterior, and opens anteriorly just under the place where the 

 inner gill-plate is attached to the visceral mass. As already 

 mentioned, the " pericardial glands " probably aid in excre- 

 tion, and possibly the same may be said of the mantle. It 

 will be noticed that the pericardium has through the nephridia 

 two indirect communications with the exterior. 



The Reproductive Organs lie in the upper part of the foot, 

 and are adjacent to the digestive gland. Ovaries and testes 

 occur in different animals, and the two sexes are distinguish- 

 able, though not always very distinctly, by the greater white- 

 ness of the testes and by slight differences in the shells. 

 The females are easily distinguished when the larvse begin to 



