No. I.] THE METAMERISM OF NEPHELIS. 33 



Professor Whitman was making a study of the nervous sys- 

 tem of Clepsine (9) for this purpose, and suggested that the 

 same be done with Nephelis, in order to bring the two genera 

 into comparison. In order that the relations between the two 

 may be clear, I present the following summary of his paper, so 

 far as it bears on this question. 



He presents some considerations drawn from embryological 

 evidence to show that the head includes a number of true 

 metameres. " Does it include anything more .■■ " 



" In the adult head we find the segments fairly well defined 

 behind the eyes, but how far the metameric division extends 

 into the prae-ocular region remains to be determined. With 

 reference to the origin of the head, we are compelled to take 

 one of two views. The head consists either (i) of a non- 

 metameric lobe plus a number of metameres originally belong- 

 ing to the trunk, or (2) of such metameres only, the non- 

 metameric head element of the ancestral form having been lost 

 or incorporated in the first metamere." 



Each body neuromere in Clepsine, disregarding the longi- 

 tudinal nerve cords which fuse regularly at the level of each 

 metameric center, comprises three pairs of nerves and si.x 

 ganglionic masses, each mass being contained in its own cap- 

 sule. Two of these are always ventral and median, the remain- 

 ing four are arranged in pairs, two on either side above the 

 nerve roots. The sub-oesophageal ganglia readily show their 

 metameric origin ; the ventral capsules of the body neuromeres 

 persist, arranged in a median row with only the two anterior 

 capsules crowded into bilateral positions. The corresponding 

 lateral capsules are readily identified in the 6th, 5th, and 

 4th segments, while the others in the 3d and 2d have been 

 crowded out of the places they would naturally occupy. The 

 nerves from this region are also identified as containing the 

 elements of the single neuromere. VI, V, and IV have three 

 roots each ; III shows only two roots, and II issues as a single 

 root, which soon divides into two branches. Sections show 

 very plainly the presence of five nerve roots, each with its pair 

 of median nerve cells. Thus the evidence is conclusive that the 

 sub-oesophageal region consists of five metameres (II to VI). 



