776 PROF. G. C. BOURNE ON THE 



figs. 19 &. 21, v.ren.), whence it passes by an afferent renal vessel 

 to the glandular portion of the kidney. So far as I can ascertain, 

 the blood is returned from the kidney to numerous small vessels 

 running in the floor of the posterior half of the mantle-cavity, 

 and is conveyed from these to the auricle by a distinct vein 

 (PI. XXXIII. fig. 20, v.post.) which opens into the hinder part of 

 the auricle, and receives in addition blood from the roof of the 

 extreme hind end of the roof of the mantle-cavity. There aie 

 thus two distinct vessels opening into the aui'icle, the foremost of 

 which brings back blood from the roof of the greater part of the 

 pulmonary chamber ; the hindmost brings blood that has passed 

 through the kidneys, then through the vessels on the floor of the 

 pulmonary chamber, and in addition a small quantity of blood 

 from the roof of the extreme hind end of the pulmonary chamber. 



The Excretory Organs. 



The topographical relations of the kidney, as compared with 

 that of the ISTeritidfe, have already been explained (p. 765). Re- 

 garded in detail, the kidney consists of a thick-walled glandular 

 portion and a thin-walled non-glandular portion which serves as a 

 bladder and urinary duct. The glandular portion is a large and, 

 roughly speaking, quadrangular sac lying in the lower part of the 

 visceral mass, below the stomach but above and somewhat to the 

 left of the lower loop of the rectum. Its posterior and left wall 

 fits closely against the pericardium and forms the inner boundary 

 of the latter. The two ends of the sac are produced into large 

 pockets or recesses, which partly extend round and embrace the 

 walls of the pericardium (PI. XXXIII. figs. 18 & 19), and partly 

 extend upwards roiand the sides of the pyloric division of the 

 stomach (Pis. XXXIII. & XXXIV. figs. 17 & 24). The cavity of 

 the sac is spacious, and only partially subdivided by folds project- 

 ing inwards from the wall on the pericardial side ; the opposite 

 wall is not folded. The renal blood-vessels run in these folds. 



The whole cavity, including the folds, is lined by a uniform 

 glandular epithelium consisting of large iri^egularly shaped cells, 

 of varying length, their free ends rounded or club-shaped and 

 often projecting far into the lumen of the sac. 



The characters of these cells are shown for Alcadia in fig. 22 

 and for Lucidella in fig. 23 (PL XXXIY.). In all the other 

 species that I have examined the kidney-epithelium resembles 

 that of Lucidella ; it is only in Alcadia that the cells are as long, 

 irregular, and amoeboid-looking as those drawn in fig. 22. In 

 both cases the cytoplasm is clear and distinctly and coarsely 

 vacuolated ; the nucleus spherical, vesicular, with a few granules 

 of chromatin. The ureter or non-glandular part of the kidney 

 arises from the upper corner of the left-hand recess of the 

 glandular sac. Its walls are composed throughout of a non- 

 ciliated, very low, cubical epithelium, the cells of which are so 

 much flattened that they might almost be called a pavement- 



