262 GILBERT C. BOURNE. 



contents are of the same colour as tlie crystalline concretions 

 which occur iu such abundance in the epithelium of the 

 stomach, I regard them as nutritive cells or trophocytes. 

 The basophile cells are possibly reserve cells or thesocytes, 

 but as I was unable to apply micro-chemical tests to my single 

 series of sections I could not determine the nature of their 

 contents. 



The Renal Organ. 



This organ is of very large size in Oligotrema, and lies 

 embedded in the ectopleromeon the right and ventral side of 

 the gut in the position shown in figs. 34, 14, and 15. It is 

 somewhat ovoid in shape, Avitli a broader anterior and a 

 narrower and tapering posterior end. Briefly described, it 

 is a closed, thin-walled sac, lined by a peculiar epithelium, 

 and its cavity is filled with a mass of formless coagulum, the 

 superficial layers of which are stratified and give rise to 

 numerous spherical, concentrically striated concretions. There 

 are no ducts and no communications of any kind between the 

 cavity of the organ and the adjacent blood-spaces, but the 

 anterior two-thirds of the sac bulge into the right peri- 

 branchial cavity, and are separated from the latter by so thin 

 a wall that the contents might easily escape by diffusion into 

 the peribranchial cavity, and so by the atrium to the exterior. 

 A closed renal sac of this kind, filled with concretions, is 

 universal in the family Molgulidfe, and has been described 

 by de Lacaze Duthiers and Kuppfer; and a similar organ has 

 been described by Van Beneden, Krohn, and others in various 

 species of Ascidia and Phallusia. In Oligotrema the renal 

 sac is exceptionally large, and is lined b}^ an epithelium 

 different from those described by the above-mentioned 

 authors, A portion of this epithelium is represented in 

 fig. 23. It consists of a layer of dark and finely alveolar 

 protoplasm, in which rathei- large, rounded nuclei are 

 embedded at intervals ; there is no trace of cell outlines. 

 This epithelium had been peeled off from the greater part of 

 the inner surface of the renal sac, and the fiagments were 



