REPRODUCTIVE ORGANS. 309 
double 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 ; (4) 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 ; (Z) 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; (ce) this mode of origin, and the 
much leéss 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 (Uzonccola 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 
