TUBIFEX. 349 



proliferation of some of its cells, is an irregularly- 

 shaped mass known as the prostate gland. The cells of 

 this gland secrete a cementing substance which is passed 

 into the spermatheca with the sperm, and is used for 

 moulding the spermatozoa into a solid mass, of 

 characteristic shape, known as a spermatophore. The 

 position which these organs occupy in the body is 

 constant, and will be noticed in the description of the 

 various organs. 



I. The Gonads. 



Both ovaries and testes are present in the same worm, 

 there being only a single pair of each. As is usual in 

 the Oligochaeta, the testes lie in front of the ovaries, 

 and in Tubifex they are situated in adjacent segments, 

 the testes in segment 10 and the ovaries in segment 11. 

 In their relative segments they occupy exactly the same 

 position in relation to the other organs present in that 

 segment; that is, they lie one on either side of the 

 intestine and slightly nearer the ventral than the dorsal 

 surface of the body. 



Not only do they correspond in their position in the 

 segment, but, in the young condition at any rate, they 

 are exactly similar in appearance and structure. Both 

 ovaries and testes are derived from peritoneal epithelium, 

 and appear in the earliest stages of their development 

 as small masses of a few undifferentiated cells forming 

 the germinal epithelium. In such a condition as this it is 

 only by noticing the segment which they occupy that 

 one can distinguish between the gonads. They are 

 attached to the septum forming the anterior boundary of 

 the segment which they occupy and hang freely into the 

 body cavity. At a little later stage, but when the cells 

 of the germinal epithelium are still undifferentiated, the 



