PATELLA. 249 



of about the right half of the visceral hump and connected 

 laterally with (b). 



(d) A sub-rectal lobe stretching beneath the rectum 

 and the back pari; of the left kidney, and with its apical 

 portion contiguous to the pericardium. 



When coloured by excretory matter or by injection, 

 the outlines of the dorsal parts of the kidney become 

 visible, making it appear to be a much branched 

 arborescent 'gland. On opening, this impression is cor- 

 rected, for we find the structure characteristic of the 

 excretory organs of Mollusca. It is essentially a sac 

 lined by renal epithelium, but the space is much 

 obstructed by the growth across it of subjacent tissue, 

 forming pillars over which the renal epithelium is, of 

 course, continued. These pillars or trabecule contain 

 extensions of the part of the perivisceral sinus in contact 

 with the kidney (see fig. 32a). They increase the 

 excretory surface, and in this way add to the efficiency 

 of the organ. It was long supposed that the blood 

 channels actually opened into the kidney, and that the 

 blood, in this way, received water ; but this idea is 

 now quite discredited, and we know that the renal 

 epithelium forms a boundary everywhere between the 

 blood sinus and the kidney cavity. This renal epithelium 

 consists of a single layer of cells of variable size. Many 

 of them are ciliated, especially when young. As they 

 grow older, these cells amass concretions, some developing 

 a number of small ones, other a few larger ones (fig. 32ib). 

 These concretions contain, mainly, nitrogenous waste, 

 and are dark brownish green in colour. The relation of 

 the kidney to the perivisceral blood sinus is that typical 

 for Molluscan kidneys generally, the renal epithelium 

 forming a much complicated partition between a blood 

 space on the one hand, and, on the other, a cavity 



