REMARKABLE SPECIES OF ECHIUROIDS. 9 



of very flat cells arranged in a single row. It is uniformly 

 developed throughout, except for a certain extent in the posterior 

 parts of the body-cavity, along the ventral median line just above 

 the nerve-cord, i.e., in the region where the male sexual organ 

 develops from the peritoneal layer. 



The Ventral Nerve-cord. — This is an unpaired, medianly 

 situated string of a considerable thickness, running in the ventral 

 body-wall for nearly, but not quite, the entire length of the body 

 {v.n.y figs. 9-13 and fig. 16, PI. II.). In the greater joart of 

 its course, the cord heaves up from below the inner surface of 

 the body- wall, thus bringing about a peritoneum- covered, broad, 

 longitudinal ridge projecting into the body-cavity (fig. IG). Only 

 in the anterior fourth of the body-length, this ridge formation 

 does not take place ; there the presence of the nerve-cord does 

 not apparently directly affect the thickness of the body-wall 

 (figs. 11-13). Anteriorly the cord terminates with two swellings 

 situated side by side and which are separated from each other 

 by an intervening lamina of the connective-tissue (fig. 10). They 

 lie in about the same level with the anterior end of the body- 

 cavity. Thus the nerve-cord does not reach up to the anterior- 

 most end of the body ; nor does it form any sort of a nerve- 

 ring, which in males of other Bonellia species is known to exist 

 around the sperm-duot. The bifid anterior termination may 

 possibly be regarded as representing the remnant of a degenerated 

 nerve-ring. 



The Alimentary Canal. — Whereas in most Bonellian males 

 hitherto known, the alimentary canal was found to be degenerated 

 to the extent that it lacked both the mouth and the anal opening, 



