BY T. HARVEY JOHNSTON AND 0. W. TIEGS. 219 



eventually comes to lie freely in the coelome in the vicinity of the 

 anterior parts of the uteri, but ultimately coalesces with the 

 dorsolateral wall of the intestine, the area of union being elongate 

 and slit-like, with greatly folded walls (Plate x., figs. 10, 11). 

 The lumen of the folded portion is narrow, and the epithelium 

 contains abundant gland cells like those of the intestine. This 

 structure does not appear to be represented in Bonellia. 



Nervous System. — The nervous system consists of a ventral 

 nerve cord and a circum-cesophageal ring. The former lies mid- 

 ventrally within the body cavity, attached to it by a short 

 mesentery. About 25 pairs of nerves are given off to the body 

 wall (Plate ix., fig.4). Ganglia are absent. Anteriorly at the 

 base of the setse the cord bifurcates, the two portions coming to 

 lie at the sides of the muscular pharynx, and being included in 

 the outer longitudinal layer of muscle, i.e., they lie in the body 

 wall. The two branches then travel along on either side of the 

 proboscis, and, entering the bifurcation, proceed to the end, then 

 turn back sharply, meeting one another so as to form an enor- 

 mously elongated circum-oesophageal nerve ring (Plate ix., fa'g.5). 

 Associated with the median bloodvessel of the proboscis, especi- 

 ally on its ventral aspect, is a mass of tissue which appears to 

 be nervous (Plate ix., fig. 6). 



Vascular System. —There is a ventral bloodvessel, which travels 

 in the body cavity immediately above the ventral nerve cord to 

 which it is attached by a delicate membrane (Plate xi., fig. 18). 

 Posteriorly, at about the middle of the body, it dilates, and, 

 leaving the ventral nerve cord, passes upwards to become applied 

 to the nearest coil of the intestine. In this region, and to a less 

 extent in other places more anteriorly, large numbers of colour- 

 less corpuscles can be detected. Posteriorly, the lumen of this 

 dilated vessel appeared in section to be more or less occupied by 

 a spongy mass resembling connective tissue, but which may have 

 been coagulated fluid. Corpuscles were present entangled in it. 

 The vessel travels backwards in close connection with the more 

 dorsal portion of the intestine (serving evidently as a food-ab- 

 sorbing organ) till it gradually disappears. The condition is 

 similar to that described by Spengel (1879) as occurring in B. 



