46 BRISTOL. [Vol. XV. 



In discussing the anal region of Clepsine, in which the same 

 condition obtains, Whitman says (9, p. 388) : " This arrange- 

 ment, evidently one of mechanical adjustment necessitated 

 by the shortening and crowding of the segments, prevails 

 throughout the caudal region with the exception of the first 

 segment (XXVIII) in which the sacs are placed one behind 

 the other as in typical trunk segments." 



Again, in his description of Clepsine plana (16, p. 413), he 

 says : " Reduction, as I have before pointed out, seems to have 

 begun at both extremities, and to have advanced from these 

 points towards the middle of the body. Its advance shows 

 how far a form has departed from the ancestral condition of 

 uniform somites. It is here that we discover a very important 

 guide to the systematic rank and relationship of different 

 forms." These seven neuromeres, then, correspond to the 

 entire anal ganglia of Clepsine, as is shown by the degree of 

 fusion in the lateral nerves and in the arrangement of the ven- 

 tral capsules. The process of reduction in the anal region has 

 gone on further in Nephelis by three metameres than in Clep- 

 sine, while in the head region the number remains the same in 

 both forms. 



The Head Region. 



In order to make an analysis of the terminal somites of the 

 head region we must keep in mind that the external criteria of 

 a neuromere are six capsules, two being ventral, two pairs of 

 nerves, and a pair of " Leydig's cells." Beginning at the pos- 

 terior end and working forwards we shall have little, if any, dif- 

 ficulty in finding six neuromeres. 



The nerves of the last neuromeres, VI (PI. VI, Figs. 10-13), 

 arise as single trunks, but divide very near the body of the 

 ganglion, and in the angle of separation of each trunk lies a 

 " Leydig's cell." Two pairs of lateral capsules, 6.6., separated 

 from the others, are easily identified as belonging to this nerve, 

 so that with the two end capsules of the two ventral series 

 we find all the elements of the typical body neuromeres. The 

 next nerve, V, arises as a single trunk and proceeds forwards as 



15 "Description of Clepsine plana." Journ. of Morph., vol. iv, 1S91. 



