BEITISH COPEPODA. 



Section II. — Pceoilostoma, Thorell.^ 



Thoeell's division of the Copepoda into three groups, 

 Gnathostoma, Poecilostoma, and Siphonostoma, the 

 distinctions between which are found in the characters 

 of the mouth-organs, is disapproved by Glaus and some 

 other authors, chiefly, as I understand them, on the 

 ground of the gradual lapse of one series into the 

 other rendering it impossible to draw perfect lines of 

 demarcation, but partly, also, on the ground of a 

 difierence of interpretation of the homologies of some 

 of the appendages. While differing from M. Thorell 

 as to the nature of some of these organs, I myself 

 think that his proposed division is a very natural 

 one, the three groups presenting characters which, 

 though differing in degree in various species, do point, 

 on the whole, to habits of life very remarkably 

 different, and deserving of expression in any natural 

 classification. 



The three groups are defined by M. Thorell as 

 follows : 



1. Gnathostoma. 

 Os mandibulis duabus libens tribusque paribus 

 maxillarum instructum, siphone nullo. 



* See p. 31, vol. i. 



