No. I.] THE METAMERISM OF NEPHELIS. 55 



blue. The bodies of the cells lie about midway between 

 the nerve rings (PI. V, long bpc), and present the same 

 general appearance in size and nucleus as the bipolar and 

 " Leydig's cells." 



The physiological role played by this highly specialized 

 peripheral system is, doubtless, of the utmost importance, and 

 anything like a discussion of its functions can be made only 

 after the details of its constitution have been more fully worked 

 out, and something is known of the comparative anatomy of 

 the structure in other worms. It plainly offers a method for 

 short reflexes, such, for instance, as those controlling the 

 rhythmic undulating motions of the leech when at rest, and 

 supposed to be respiratory ; or for the successive stimulation 

 of the muscles in voluntary motions. The presence of this 

 system may throw some new light on the phenomena that go 

 under the name of "skin tension." 



Leaving its physiological functions for another investigation, 

 its presence may be brought to bear testimony on the question 

 of the derivation of the five-ring form of metamere. In his 

 Metamerism of Clepsine, Whitman says (p. 392) : " In my 

 description of Clepsine plana (1891) the following note may be 

 found (p. 414): ' I am reminded of an error into which I fell 

 in my paper on Japanese leeches. The error was the assump- 

 tion that all somites having less than five rings were abbre- 

 viated. The assumption should have been, as I now feel 

 convinced, that all somites with less than three rings are 

 abbreviated, and all with more than three have been increased 

 by the division of one or two of the three primary rings. I 

 have collected considerable evidence, which cannot be given 

 here, to show that in the evolution of Hirudo it was the 2d 

 and Jd rings that tmderwent division, while the ist remained 

 undivided.' " On page 393 he continues, under the head of 

 " Multiplication of annuli " : " It is a fact of some importance, 

 in estimating the morphological value of the metamere, that 

 the multiplication of annuli seems to follow the same general 

 law as the multiplication of metameres in the embryo ; that is 

 to say, the posterior end of the metamere, like that of the 

 embryonic trunk, is the region of most rapid growth and elon- 



