18 bulletin: museum of comparative zoology. 



length. The head is thread-like and 7 jx long, and is to be distinguished 

 from the tail only by the fact that it stains a little more deeply. 



Kerbert ('8l) and Looss ('85) both describe peculiar semilunar or 

 crescentic bodies which contain one or more nuclei and from which num- 

 bers of spermatozoa extend. Kerbert thinks that they probably represent 

 sperm-morulse in their last stage of development, and Looss is inclined 

 to accept his explanation. Wright and Macallum found structures hav- 

 ing a similar appearance. These are caused, they state, by a sheaf of 

 spermatozoa being seen over a nucleus of one of the primitive germ cells, 

 the chromatin of which is accumulated at our pole in the form of a 

 cap. This, however, would not explain the nuclei seen in the crescents by 

 Kerbert and Looss. The only thing I have observed that could suggest 

 such a structure is the sperm-morula from which the tsils of a part of 

 the spermatozoa have escaped, the spermatozoa remaining within the 

 cytoplasm continuing to be arranged in a cap-like mass. This would give 

 the appearance of a crescent (Plate 3, Fig. 26). In Hemiurus there 

 is nothing in these structures to correspond to the nuclei of the crescents, 

 though in the forms in which nuclei have been observed it is possible 

 that some of the nuclei of the sperm-morula remain without developing 

 into spermatozoa. It seems possible, then, that Kerbert is correct in 

 regarding the structures he observed as sperm-morula? in their last 

 stage of development, and that the trematodes in which they have been 

 observed differ from Hemiurus crenatus in that this cytoplasm of the 

 sperm-morula in the former takes on a saucer-like form before the tails 

 of the spermatozoa become free from the cytoplasm and that some of the 

 nuclei of the sperm-morula do not develop into spermatozoa. 



From each testis there leads forward a very narrow duct, the vas 

 deferens. The apparently structureless walls of these ducts is continuous 

 with that of the testis. The two ducts open separately into the posterior 

 end of the semhial vesicle (Fig. 1, vsl. sem.), which curves over the 

 dorsal surface of the ventral sucker. The size of the seminal vesicle 

 varies considerably, according to the number of spermatozoa which it 

 contains. Its posterior end is usually at a point dorsal to the centre 

 of the ventral sucker, and its anterior end often lies at a point dorsal 

 to the pore. The vesicle may, however, extend no farther forward than 

 the anterior margin of the ventral sucker, or it may be so enlarged as to 

 extend to a point dorsal to the pharynx. The width of the vesicle is from 

 one-tenth to one-third that of the body. Numerous small flattened nuclei 

 are sometimes to be seen in its wall (Plate 3, Fig. 28), and it seems to be 

 provided, especially in its terminal portion, with circular muscle fibres. 



