476 



middle and not the anterior ring of the 3-annulate leech somite and 

 its derivatives. 



Oka, however, has come to a different conclusion. He compares 

 the complete somite of Ozobranchus and Glossiphonia and, while re- 

 cognizing that the large annulus of the former is morphologically 

 equal to two annuii of the latter, believes that these two are the first 

 (neural) and second (post-neural) instead of as Castle (1900) and I 

 believe the neural and its predecessor. 



The case becomes still clearer in favour of the latter view when 

 comparison is made on either hand with a form of intermediate struc- 

 ture. This is found, in a, as yet undetermined, species of fresh-water 

 Ichthyobdellid in which the complete somites are divided into three 

 annuii, but the furrow separating a\ from a 2 is much more shallow 

 than that between a 2 and ad. The nerve ganglion, metamerie sen- 

 sillae, pulsating blood vesicles and more posteriorly the nephridial 

 openings are situated within al which evidently corresponds to the 

 posterior half of, and with the associated a 1, to the entire large annu- 

 lus of Ozobranchus Mendiesi. 



Much the same condition as that last indicated is found in a spe- 

 cies of Ozobranchus which Apathy (1890) has described under the name 

 of Pseudobranchellion Margoi. This species is described as tri-annu- 

 late and Blanchard ('94) states that the preneural ring is double in 

 large individuals, making four in all if these are counted as two. 

 Apathy, however, after stating that the furrow between ring 3 and the 

 succeeding ring 1 of his enumeration is more shallow than the furrows 

 which separate ring 2 from 1 and 3, says (p. 125). »Dieses Ver- 

 hältnis ist besonders auf den kiementragenden Somiten auffallend, 

 wo die Furche zwischen dem 3. und 1. Ring beinahe ganz ausgeglättet 

 ist. Das Hauptseptum des Somits liegt zwischen dem 2. und 3. Ring 

 und bedingt hier die tieferen Furchen (Einschnürungen) des Körpers.« 



If the »Hauptseptum« is the true coelomic dissepiment, which 

 some will dispute, its position as described by Apathy corresponds 

 exactly with the neuromeric limits of the somite, and the relative 

 prominence of the inter-annular furrows is just what the neuromeric 

 standard of Castle would lead one to expect. In the application of 

 such a standard the rings enumerated as a 1, a 2 and a 3 would corre- 

 spond respectively to Apathy's 3 rd , 1 st and 2 nd rings. 



Papers cited. 



Apathy, S., 1890. Fseudobranchellion Margoi (Nova familia Hirudinearum). Er- 

 tesitö az Erdélyi Museum Egvlet. Orvos-Természettudomânyi Syakosz- 

 tâlyâbol. Vol. XV. p. 122—127. 



