TUBIFEX. 371 



outer sheath is provided with muscles externally, these 

 being disposed mainly in two ways (PL V, fig. 22). 

 There is a comparatively delicate layer of circular muscles 

 completely investing the sheath, and, from this layer, 

 oblique muscles pass, in a somewhat irregular fashion, to 

 the body wall. Certain of these fibres branch, and the 

 branches anastomose with the longitudinal muscles of 

 the body wall. 



The cavity of the penis proper is surrounded by a 

 single layer of elongated cells (PI. VII, fig. 45, le.) which 

 encroach considerably on the lumen. Near the apex of 

 the penis they are so large that they almost obliterate the 

 lumen altogether. 



V. The Development and Structure of the Ova. 



The ovary of a young worm consists of a solid mass 

 of undifferentiated cells, of which those which are nearest 

 to the septum to which the ovary is attached are the 

 youngest. The nuclei of these cells are often to be seen 

 undergoing mitotic division, and by this means the 

 number of the cells making up the ovary is increased. 

 The first formed cells are gradually pushed further away 

 from the septum, and these give rise to potential ova or 

 oogonia, which can be recognised by the large size of 

 their nuclei as compared with the amount of cytoplasm 

 surrounding them. 



Vejdovsky suggests that in such a form as 

 Bhynchelmis the oocytes are at first amoeboid, and that 

 their subsequent rapid increase in size is due to the 

 ingestion of the ova around them. I have never been able 

 to identify amoeboid oocytes in Tubifex, but some sections 

 have revealed what appears to be an equally effective 

 mode of nutrition. The young ovary is, as has already 



