CONTRIBUTIONS FROM THE ZOOLOGICAL LABORATORY OF THE 

 MUSEUM OF COMPARATIVE ZOOLOGY AT HARVARD COLLEGE. 

 E. L. MARK, DIRECTOR. — No. 108. 



THE METAMERISM OF THE HIRUDINEA. 

 By W. E. Castle. 



Received January 20, Presented by K. L. Mark, February 14, 1900. 



CONTENTS. 



I. Introduction 285 



II. Limits of the Somite- in tlic 



Rhynehobdellidae .... 286 



1. Neuromeres as a Criterion of 



Somite Limits 280 



a. Somite Reduction in Glos- 



eiphonia 287 



2. Septa and other Metameric 



Organs as Criteria of So- 

 mite Limits 290 



III. Limits of the Somite in the 



Gnathobdellidse 291 



1. Relation of Five-ringed to 



Three-ringed Type of So- 

 mite . 291 



2. Somite Reduction in Nephe- 



lis 293 



IV. Somite Growth 290 



1. Rhynchobdellidae .... 297 



2. Gnathobdellidae 299 



V. Primitive Condition of the 



Leech Somite 300 



Summary 302 



Papers cited 303 



I. Introduction. 



As long ago as the year 18G2 Gratiolet pointed out that in the 

 medicinal leech, Hirudo medicinalis L., the successive rings of the body 

 are not till alike, but that they fall into similar groups of five each, which 

 are repeated in regular succession throughout the greater part of the 

 body. He found in each such group a single nerve ganglion and a single 

 pair of nephropores ; in other words, he discovered that there was a 

 segmentation of internal organs (metamerism) which agreed not with 

 single external rings but with the groups of rings. 



Gratiolet called the groups of rings " zoonites " (somites). He noticed 

 on the exterior of the body two natural landmarks, which indicated to 



