84 ALPfiED GIBBS BOURNE. 



tliat unless disturbed in some way the "burrow'^ remains as a 

 permanent structure. The mud can be dried in cakes, and 

 uhen such a cake is broken across, the "burrows" areas 

 obvious as they are in a lump of earth in which earthworms 

 have lived. 



Some of these mud-living Oligochaeta secrete a glutinous 

 substance, so that when they are removed and the loose par- 

 ticles washed away a tube of mud remains, while others secrete 

 no such substance ; and when these latter are removed and 

 the loose mud washed away the worm remains quite unpro- 

 tected. Chsetobranchus belongs to the latter category. 



External Characters — Branchial Processes — Seta3. 

 — The worms vary in size, but when stretched out and crawling 

 on a slide an average sized individual is about IJ inches to 2 

 inches in length, and about Jjy inch in breadth (fig. 9), and 

 consists of about 130 segments. The anterior extremity is a 

 little thinner than the rest of the worm, and the body wall 

 in this region is slightly pigmented ; the pigment is to a cer- 

 tain extent arranged in transverse bands on the dorsal surface, 

 each band corresponding to a segment. There is no proboscis, 

 and the eye-spots are absent. 



The most remarkable and striking feature of the worm is the 

 presence of dorso-laterally placed processes, of which there is 

 a pair to each of the anterior segments, commencing with the 

 second^ segment. It is difficult to say how many pairs of these 

 processes exist, as, after the ten to twelve most anteriorly 

 placed pairs, they gradually diminish in size until they become 

 mere warts on the surface of the worm, and in the posterior 

 segments are entirely absent (fig. 1). I have counted about 

 sixty to seventy pairs. In most of the individuals I have exa- 

 mined the five or six most anteriorly placed processes are a 

 little shorter than those immediately following, but there is no 



^ I have assumed lliat the first setigerous segment is the second segment 

 of the body, the first segment of the body being the buccal segment with the 

 prostomium. The nomenclature of the segments in earthworms is usually 

 based upon this assumptioiij and it would be more convenient if a similar 

 practice were always adopted with regard to other Oligocha^ta. 



