TT7BIFEX. 355 



find in the same sperm smear most of the stages in the 

 development of these spermatozoa, but in other cases they 

 seem to occur much more rarely. 



At first I tried to incorporate the stages of develop- 

 ment described above in my description of the development 

 of the normal spermatozoa, but when I was able to get 

 such a complete series of each I gave up the attempt. I 

 now suggest that this worm possesses dimorphic sperma- 

 tozoa, a condition which has been described as occurring 

 in Paludina amongst the Mollusca and in certain 

 Amphibia, Birds and Mammals. In Paludina the two 

 kinds of spermatozoa are different in shape, the normal 

 one being divisible into a spirally coiled head and an 

 extremely long tail. The larger kind is vermiform, and 

 bears a tuft of cilia at one end. In Birds and Amphibia 

 the two kinds of spermatozoa are of the same form, but 

 differ in size, the smaller one being functional. 



In Tubifex rivulorum both kinds of spermatozoa are 

 elongated structures, and each is divisible into head, 

 middle piece, and tail, but the proportions of these parts 

 to each other in the two forms are rather different 

 (PI. VI, fig. 40). The head of the normal spermatozoon 

 is small, forming about one-sixth of the total length 

 (PI. VI, fig. 40a); it is oval in shape with a sharply- 

 pointed anterior extremity, and measures about 4/* in 

 length. It is composed almost entirely of nuclear 

 substance, there being such a small quantity of cytoplasm 

 present as to be almost negligible. The middle piece, 

 which is shorter than the head, lies behind that 

 structure, and gives attachment to the tail. It is composed 

 of cytoplasm and stains only slightly with nuclear stains, 

 such as haematoxylin. The tail is very long and delicate, 

 and is also composed of cytoplasm. The other spermatozoa 

 are much larger, and the head forms about half the whole 



