NOTES ON THE ANATOMY OP DINOPHILUS. 47 



direction, so as to form a pair of narrow, horizontally placed plates 

 of cells, still separate from one another. The penis is already 

 developed as a hollow mass of cells attached in its definitive position 

 by a narrow stalk to the ventral ectoderm of the body. There is 

 no connection between the testes and penis, nor conld any vesiculse 

 seminales be identified with certainty in the sections on which the 

 observation of this stage was made. As development proceeds, the 

 lateral extension of the testes goes on increasing, and the two origi- 

 nally separate rudiments fuse from place to place across the middle 

 line. The testis now consists of a solid plate, composed of a few 

 layers of cells, extending along the ventral side of the stomach, and 

 still showing obvious traces of its double origin. The testis next 

 extends laterally round the stomach, still composed of a solid mass of 

 cells. In the final condition, some of these sperm mother-cells are 

 found in groups in various parts of the testis, whilst ripe and half- 

 ripe spermatozoa are found moving about freely in the indefinite 

 cavity which is by this time excavated in the interior of the organ. 

 The testis is separated frorn the body-cavity by a distinct mem- 

 brane. 



Although, in the adult condition, the testis is constantly continuous 

 across the middle line in its anterior and posterior regions, it is 

 usually divided into two lateral halves, in the region of the aperture 

 from the stomach into the intestine, by a median extension of the 

 body-cavity, which, as already explained, contains the internal ends 

 of the fourth nephridia. The testis, in its most fully developed form, 

 extends from the region of the muscular appendage of the oesophagus 

 nearly as far as the anus, as shown in fig. 15. 



Unripe spermatozoa are found, attached together in sperm-morulse, 

 in the cavity of the testis. The fully developed spermatozoon (fig. 

 4) is an extremely long, actively moving, undulating fibre. It hence 

 closely resembles in form the spermatozoon of D. vorticoides as de- 

 scribed by van Beneden (1) and Mereschkowsky (10), excepting that 

 Mereschkowsky describes and figures a swollen head in the sperma- 

 tozoon of -D. vorticoides. I believe that no such structure occurs in 

 D. tseniatus, although at the time when fresh material was accessible 

 to me I was not familiar with Mereschkowsky' s paper. 



Although ripe spermatozoa may be found in any part of the adult 

 testis, they are always present at its posterior end, if they have any- 

 where reached their mature condition. As has been already ex- 

 plained, the testes are fused together across the middle line in the 

 region of the fifth body-segment, and the ripe spermatozoa which 

 accumulate in this part of the organ are taken into the interior of a 

 pair of vesiculse seminales {v. fig. 15). In their most fully deve- 

 loped condition these structures are much larger than in the figure 



