ON THE HKAD KIDNEY OF BDELLOSTOMA. 173 



During the summer of last year, Mr. Sedgwick, while visiting 

 the Cape of Good Hope, collected amongst other things a large 

 number of very fine specimens of Bdellostoma Forsteri, 

 var. hexatrema ; and on his return to Cambridge he very 

 kindly obtained permission from the Royal Society, for whom 

 the specimens were collected, to allow me to examine their renal 

 organs. On making a superficial examination of the so-called 

 head kidneys, it was evident, as shown in fig. 1, that 

 they were separated by a considerable distance from the 

 anterior end of the segmental duct {s. d.), the only structures 

 passing from one organ to the other being apparently blood- 

 vessels. The subsequent preparation of a complete series of 

 sections, the first of which passed through the anterior ex- 

 tremity of the '' head kidney," the last through the beginning 

 of the segmental duct, proved conclusively that with the ex- 

 ception of a rudiment to be spoken of presently, no trace of 

 connection existed between the two organs. 



Transverse sections showed the presence of a number of 

 branched ducts, evidently the ^'pronephric tubules '' of Wilhelm 

 Miiller, which opened on the one hand into the pericardium, 

 and on the other into a central duct (figs. 3, 7). 



These tubules (PI. XV, fig. 7) had an average diameter of 

 6 mm. ; there was no increase, but rather a diminution in 

 diameter at the openings into the pericardium (figs. 2, 3, 7, /.). 

 Each tubule was lined by cubical or columnar cells, the 

 protoplasm of which Avas finely granular ; each cell contained 

 a large, elliptical, highly refracting nucleus, containing nu- 

 merous coarse dark granules (fig. 7). At the mouth of each 

 tube the columnar lining epithelium was continued into the flat 

 pericardial epithelium (fig. 7, jo. c). No traces of cilia were 

 found on the cells bounding the openings into the pericardium. 

 Outside the lining epithelium was a well-marked basement 

 membrane (fig. 7, b. m.); and outside this, in the spaces 

 between the tubules, was a small quantity of connective tissue, 

 and an exceedingly rich plexus of blood-capillaries (fig. 7, b. c.) ; 

 so that during life a very considerable quantity of blood must 

 be constantly passing between the tubules of the gland. 



