356 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



structure. The head is elongated, but less sharply pointed 

 at the anterior end than is that of the normal sperma- 

 tozoon (PL VI, fig. 40b). The middle piece is less 

 distinctly marked off from the head than in the first case, 

 and the tail is straight throughout. 



III. The Sperm-sac 



The sperm-sac is developed, as in all Oligochaeta, 

 for the reception of the spermatogonia, which are early 

 removed from the testis and transferred to this organ. In 

 it the mature spermatozoa are formed. 



In a worm in which the reproductive organs as a 

 whole are immature, the sperm-sac can be seen in its 

 simplest condition. It is an unpaired structure, and arises 

 as a pouch-like outgrowth of the septum between segments 

 10 and 11. This outgrowth is directed backwards and 

 projects into segment 11. The cavity thus formed is at 

 first quite simple, and is bounded by a layer of flattened 

 peritoneal epithelium which is derived from the tissue of 

 the septum. As the number of spermatozoa increases, 

 the sperm-sac also enlarges, and it soon becomes too big 

 for the segment in which it developed, and projects still 

 more posteriorly into segment 12. In a mature worm the 

 number of spermatozoa is very large, and these are present 

 in the sperm-sac in all stages of development. The 

 sperm-sac increases tremendously in size, extending 

 further and further back until it may occupy as many as 

 seven or eight segments (PL I, figs. 2 and 3, sp.s.). Ii 

 is the presence of this large sperm-sac which gives the 

 mature worm a swollen and opaque appearance in the 

 region of the reproductive segments. It is very common 

 also to find that the sperm-sac has encroached upon the 

 segments in front of that occupied by the testis, and 



