544 



PROCEEDINGS OF THE NATIONAL MUSEUM. 



Whitman. He regards the multi-annulate somite as primitive. The 

 view here supported is that the primitive typical leecli somite con- 

 sisted of three aimuli. These primary aiimili can readily be recognized 

 in all families and most species of leeches which I have examined. 

 When the primitive tri-annulate character of the somite is lost this 

 may take place by (a) reduction, which has occurred as a result of 

 coalescence of the primary rings at the anterior and posterior ends of 

 nearly all leeches and in the genital regions of some, or {}>) by elabora 

 tion, which has taken place in the somites of the middle body region, 

 especially of the (InathobdellidjL', Herpobdellid{e,and IchthyobdoUidie. 

 The increase in the number of annnli by which this elaboration is 

 expressed externally seldom if ever occurs by the actual intercalation 

 of new rings, but only by the growth and lesser or greater subdivision 

 of the three primary rings. This subdivision seems to follow a regular 

 law, which is that any number or all of the primary rings may become 

 secondarily biannulate, the secondary annuli similarly biannulate 

 and the tertiary again divided for the fourth time, and any one 

 of these subdivisions may be in various degrees partial or complete, 

 and may aiTect one or more annuli of any order. The theoretical com- 

 ideteness of the process is expressed in the following table, which also 

 presents a system of nomenclature for the maximum possible number of 

 annuli of each order, up to the fourth, of a complete somite, enabling 

 tlie structure of a typical somite of any genus to be expressed by a 

 simple formula. 



Table of annulations. 



First 

 order. 



Second 

 order. 



Third 

 order. 



a2 



, a3 



Total. 3 



Fourth 

 order. 



cl2 



d\ 

 d2 

 d3 

 di 

 d5 

 d6 

 dl 

 di 

 d9 



(UO 



dll 

 dl2 

 dl3 

 dU 

 dl5 

 dl6 

 dl7 

 dl8 

 dl9 

 d20 

 d21 

 d22 

 d23 

 d24 



12 



24 



1 give a few illustrations of the application of the system. Proto- 

 clepsine and many other Glossiphonidai have a simple tri-annulate som- 

 ite ai + a2 -\- aS. Many of the larger species of Glossiphonia show a 

 slight subdivision of the second and third primary annuli, which be- 

 comes strongly expressed in Hcementaria, al -^ a2 {bSb^) + a3{b5b6). 



