4 BRITISH COPEPODA. 



mediate between the true biting and chewing 

 apparatus of the free swimming species, and the 

 true suctorial mouth of the fish-lice and other 

 thorough-going parasites. From a consideration of 

 these peculiarities M. Thorell has proposed to arrange 

 the Copepoda under three sections — Gnathostoma, 

 Pcecilostoma, and Siphonostoma — which arrangement 

 though adopted by Claparede and others is rejected by 

 Claus chiefly, as it appears, owing to a difference of 

 opinion as to the homologies of the various mouth 

 organs. In this work I adopt the three divisions 

 proposed by M. Thorell, because whatever opinion 

 may be held as to the nature of the component parts 

 of the mouth apparatus, each division seems to repre- 

 sent a well-marked natural group. Some remarks on 

 this part of the subject will be found at greater length 

 under the definition of the group Pcecilostoma. 



Throughout the whole class Crustacea the mouth 

 organs, and other cephalic appendages, are liable to the 

 most profound modification according to the needs 

 and circumstances of particular groups, and on this 

 account the conformation of these parts offers a 

 most natural and trustworthy guide to classification. 

 In no order is this more conspicuously the case 

 than in the Copepoda, which, from the lowest and 

 absolutely inactive Epizoa,* to the most agile and 

 highly organised of the free swimming Calanidse, offer 

 an endless variety of structure. So rudimentary, 

 indeed, in development, are some of the lower Epizoa, 



* These, however, are often separated from the Copepoda to form an 

 independent order. 



