Sfructure of the Oligochxeta. 



93 



ment 12. The atrium commences as a sinuous tube, which 

 widens out to form a large thin-walled sac with muscular 

 walls. This sac when cut open (see fig. 1) is seen to be nearly 



Fig. 1. 



Sperm-duct of Phreoclrihis. 

 A, atrium ; /, its junction with vas deferens ; ;, junction of the latter with 

 csecal appendage (.S))) ; v.d, vas deferens; p, external pore of atrium. 

 The segments are numbered. 



filled with a much coiled continuation of the atrium and the 

 vas deferens. The vas deferens makes its exit from the 

 atrium at a point nearly opposite to its entrance ; just before 

 this point it gives off {j in the figure) a diverticulum which, 

 after being bent several times upon itself, ends blindly in the 

 neighbourhood of the funnel in wliich the vas deferens ter- 

 minates. The periatrial sac is filled with ripe spermatozoa 

 not indicated in the woodcut. It is not, however, as far as I 

 can ascertain, a coelomic sac ; its cavity is simply produced by 

 a splitting ofJF of the greater part of its muscular tissue from 

 the atrium. 



How the spermatozoa find their way in, unless it be through 

 the gaps between the individual fibres, I cannot imagine; 

 neither have I succeeded in finding any communication be- 

 tween the sac and the interior of the vas deferens. The 

 diverticulum of the vas deferens is lined with a 7jo/i -ciliated 

 glandular epithelium and has a muscular covering ; its struc- 

 ture is indeed precisely that of the unusually elongated 



