ON PHYMOSOMA VARIANS. 15 



whole process is very similar to what takes place in a mammary 

 gland during the excretion of milk. Theel mentions that the 

 excretory organs of Phascolion emitted yellow vesicles which 

 resembled drops of oil when the living animal was disturbed.^ 



Between the papillae lie a series of hemispherical depressions 

 lined by a flattened epithelium^ the cells of which are usually 

 loaded at their base with the yellow granules above men- 

 tioned. These cells seem to develop into the high columnar 

 cells described above. 



The muscle-fibres form an irregular network outside the 

 nephridial cells, lying chiefly at the bases of the papillse. The 

 hemispherical depressions seem to pass through the meshes of 

 the muscular coat, and to lie in direct contact with the perito- 

 neal investment of the organ (figs. 19 and 20), forming a series 

 of projections visible on the external surface. 



The peritoneal epithelium which surrounds the kidney is dis- 

 tinguishable from the nephridial cells by the greater ease with 

 which it absorbs staining fluids, and by the absence of secretion 

 granules. In the region of the hemispherical depressions 

 the peritoneal cells frequently form thick masses several cells 

 deep. 



It is difficult to avoid the conclusion that the excretion pro- 

 ducts are passed through the peritoneal cells to the cells of the 

 hemispherical cups, and thence to the cells of the papilla?, 

 the internal opening of the nephridium having relation chiefly 

 to its function as a generative duct. 



The relative amount of the secreting epithelium to the cubical 

 epithelium lining the bladder varies greatly ; in one specimen 

 even the area between the external opening and inner end of 

 the internal opening was lined with the former cells, thus 

 reducing the bladder to a very small structure. 



The lumen of the nephridium contains nothing but the 

 vesicles above described, together with ripe ova or spermatozoa. 

 It is remarkable that the ca4omic corpuscles appear never to 

 pass through the internal opening of the organ. 



> Theel, " Recherches sur le Phascoliou strombi," ' Kougl. Svenska Ve- 

 teuskaps-Akademiens Handlingar/ Bandet 14, No. 2. 



