102 



MK. E. C. BOUSFIELD ON THE 



only be so called for convenience ; since for the greater part of 

 its length it runs along the side or lower part of the intestine 

 embedded in its wall, only becoming free when it reaches the 

 junction of the latter with the stomach. Prom this point it is 

 connected by a varying number of contractile loops Tvith the 

 ventral vessel, and terminates exactly as in Nais. In the bran- 

 chial area the circulation between the dorsal and ventral vessels 

 is carried on as follows : — The abdominal vessel, running back- 

 ward undivided to the boundary of the mucous layer of the area, 

 divides into two branches which run right and left round its 

 margin. Each of these main branches gives off at least three 

 secondary ones, one to each of the branchia of its own side, which 

 runs to its summit and down again without breaking up into a 

 capillary network ; whilst the third, also undivided, runs obliquely 

 across the area, the continuation of the main branch running on 

 round the margin. All these branches again unite to form a 

 common trunk, which joins with a similar one from the opposite 

 side to form the commencement of the dorsal vessel. 



The specialization of the termination of the hinder portion of 

 the intestine does not appear to have any effect in abrogating the 

 general respiratory function of that tube ; doubtless an advantage 

 to these worms, which spend so large a portion of their lives in 

 crawling in the mud in search of food. It should be remarked 

 that the tube is much too narrow to admit of the branchial area 

 being drawn into it when expanded ; nor is the tube wide enough 

 to give room for the bristles, the latter being always much curved 

 when within it ; and though the worm is able, as a rule^totarn 

 round with facility, yet it sometimes happens that it becomes 

 fixed in the tube in so doing, and is quite unable to extricate 

 itself. 



Distribution of Species, 



With the exception oiDero Miilleri, received from Mr. Bolton, 

 of Birmingliam, I have found all the known forms within a short 

 distance of London. The richest hunting-grounds are without 

 doubt the tanks at Kew and at the E-oyal Botanic Society's 

 Gardens, Begent's Park. The latter locality has furnished a 

 species, the only one whose indigenous character is doubtful, viz. 

 the hesiutihil D. furcata, the finding of which and the difficulty 

 of obtaining any reliable information about it, first led the writer 

 to undertake the study of this genus. Here also he first found 



