REPRODUCTIVE ORGANS. 399 



doable 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 muscle- 

 fibres, a narrow zone of circular muscle-fibres, an internal 

 epithelium, and the two canals. The white circular muscle- 

 fibres are unstriped; the longitudinal muscle-fibres, which 

 are greyish yellow, show a lozenge-shaped marking as in the 

 more opaque fibres of the adductor muscles. 



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- 

 sect:' on 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 (Unionicola or Atax ypsilophorus). 



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 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 excretion, and possibly the same 

 may be said of the mantle. 



The reproductive organs. These lie in the upper part 

 of the foot, adjacent to the digestive gland. Ovaries and 



27 



