Southern — Monograph of the British mid Irish Oligochceta. 145 



of the intestine. In No. 3 the section cuts the dorsal vessel and the obliquely- 

 lying sacs separately. There are slight indications at this point of a blood- 

 sinus surrounding the gut. On passing from the tail towards the middle of 

 the body this sinus becomes more prominent. About the middle of the body 

 (fig. 8, e), the dorsal vessel appears only as the dorsal contractile portion of a 

 perivisceral sinus which receives blood from the ventral vessel, and sends it 

 forward in the dorsal vessel. This interpretation of the structure revealed by 

 transverse sections is confirmed by examination of the living worms. In 

 optical section the intestine shows a diffuse but distinct reddish tint, which is 

 most strongly marked in the middle of the body, and which is evidently 

 caused by the blood in the perivisceral sinus. The course of the blood is 

 evidently as follows. In the anterior region it passes from the dorsal vessel 

 through the commissures of the first 13 segments into the ventral vessel. 

 In the latter it runs backwards, gradually passing through the median 

 branches into the intestinal sinus. These branches, as already stated, are 

 very numerous in the tail. Here they probably form a plexus round the 

 intestine ; and from this plexus alone the dorsal vessel is formed. There are 

 no integumental vessels. Passing forwards the vessels of the plexus fuse to 

 form a sinus round the intestine, and in open communication with the 

 dorsal vessel. It is a debated point whether a plexus or sinus is present 

 round the gut in the Oligochseta, but in this case there seems to be no doubt 

 that a sinus is present. That the dorsal vessel is fed from the intestinal sinus 

 along the greater part of its length is also proved by the fact that when the 

 worm is cut into two pieces at any part behind the clitellum, the dorsal vessel 

 still continues to receive blood and to pulsate. 



In the family Lumbriculidse there is great variety of structure in the 

 vascular system. There is no species, however, which at all resembles the 

 one just described. The structure of the reproductive organs clearly proves 

 that it belongs to the genus Stylodrilus. The restriction of the blind sacs to 

 the tail and their simplicity of structure, considered in conjunction with the 

 fact that they are quite absent in the other species of the genus, seem to show 

 that they are undergoing a process of elimination. Taking into consideration 

 the importance of the tail for purposes of respiration in these aquatic worms, 

 it is natural that the blood-glands should be retained here when they have 

 disappeared from other parts of the body. The new species thus forms an 

 interesting link between the normal Lumbriculid type and the aberrant genus 

 Stylodrilus. 



Mature — April, May, June, 



HaMtat — Ireland. E. Annalee, Ballyhaise, Co. Cavan ; Pond on moor, 

 Carrickmines, Co. Dublin ; Lough Bray, Co. Wicklow. 



