SPECIES OF THE GENUS AULODRILUS, 



959 



are high and low at regular intervals, and swollen at their inner 

 projecting borders, which here lie outwards ; the layer of muscle 

 fibres outside the epithelium in the atrial duct forms here the 

 inner wall (text-fig. 5). In the centre of the penis is a tube — a 

 part of the a.trial duct, the wall of which is continuous with that of 

 the outer tube at the terminal opening. It is obvious therefore, 

 that the penis is here formed by the eversion of the terminal 

 portion of the atrial duct carrying with it the more proximal 

 part as the central, tube. The naiTOW proximal poi-tion of the 

 atrial duct does not seem to be draAvn into the penis. The 



Text-figure 5. 



Part of transverse section of the body tlirougli the penis, coelomic sac, 

 and a part of the atrium and prostate. X 390. 



eversion of the atrial duct is in all probability brought about by 

 the contraction of the muscle fibres forming the muscular sac, 

 which in turn are connected with the muscular layer of the body- 

 Avall. The penis in these worms is not a permanent structure and 

 hence is very different from that in many Tubificids ; it should be 

 regarded as a pseudo-penis. 



The sperm-sac is a median pouch formed by the extension 

 backwards of septum 6/7 ; it opens anteriorly by a wide mouth in 

 the 6th segment, and lies generally dorsal to the oesophagus in 

 the 7th and a part of the 8th segments. Its walls are as thin as 



