132 NOTES ON THE ANATOMY OF DINOPEILUS. 



by the following considerations : (i) That no other species of Dino- 

 philus is known to be liermaphrodite ; (ii) that the process of ferti- 

 lization was frequently observed in D. tseniatus ; (iii) that the 

 spermatozoa so constantly seen in the female of the same species 

 were, without exception, ripe and actively moving, no trace of sperm- 

 morulse or unripe spermatozoa being discernible. Such stages in 

 the development of the spermatozoa were never missed in any adult 

 male individual. 



It will be noticed that the above-described process of copulation 

 in D. tseniatus exactly resembles the processes which have been de- 

 scribed by Lang (8, p. 231) in certain Polyclada [Anonymus, &c.) 



The morphology of the vesicula3 seminales is one of the most 

 interesting features of D. tseniatus, since there is reason to believe 

 that these structures are the modified fifth nephridia of the male. 

 The reasons for this conclusion are two : 



(i) Five pairs of ordinary nephridia occur in the female D. 

 tseniatus (as in the female D. gyrociliatus) , whilst the most careful 

 examination, often repeated, of the males of the same animal failed 

 to show any trace, in that sex, of the existence of a fifth pair of un- 

 differentiated nephridia. 



(ii) The consideration of young stages of the vesiculse seminales. 

 Fig. 5 represents the earliest of these stages which was observed. 

 The vesiculse seminales were in their definitive position in the fifth 

 body- segment, and their indentification as vesiculae was rendered 

 sufficiently certain by the fact that they contained ripe spermatozoa. 

 The vesiculse were arranged in an obliquely transverse position, their 

 outer portions ending blindly at a level between the two ciliated 

 rings of the fifth segment, their inner ends opening into the cavity 

 of the testis. A part of the vesicula immediately succeeding the 

 internal aperture was lined with long cilia ; the next part of the tube 

 contained a small mass of spermatozoa. The penis was well deve- 

 loped, and obscure indications of a duct leading from the vesicula to 

 the penis were observed ; the existence of this duct was not, how- 

 ever, completely proved. The resemblance of the young vesicula 

 seminalis to an ordinai-y nephridium was manifested, not only in its 

 shape and position, but still more conspicuously by the fact that its 

 walls contained an orange pigment, exactly resembling that so com- 

 monly found in the walls of the excretory tubes. 



Stages intermediate between that represented in fig. 5 and the 

 mature foim of the vesicula seminalis were frequently observed. 

 The final form is acquired by the gradual distension of the originally 

 subcylindrical tube by spermatozoa, this distension being accom- 

 panied by an alteration in the direction of its axis, the result of 

 which processes is that the end which, in the young vesicula, is 



