2 BRITISH COPEPODA. 



the professed limits of my work, and have extended 

 this preface beyond reasonable bounds, — on which 

 account those species are noticed in the most cursory 

 manner, and only in elucidation of the proper subject 

 of the memoir. 



The non-parasitic Copepoda may be described as 

 Entomostraca, having an elongated body of variable 

 form, but generally cylindrical, without a bivalved 

 shell, and showing more or less completely a division 

 of the body into numerous rings, or somites. There 

 are two pairs of antennse, three pairs of prehensile 

 and masticatory, or suctional, mouth organs, and five 

 pairs of feet, adapted chiefly for swimming. The 

 females are mostly fertilized by means of spermato- 

 phores ; the ova are usually carried in external ovisacs ; 

 when first hatched the larvse have only three pairs of 

 limbs, and go through several metamorphoses before 

 attaining the mature form. 



The parasitic species, at one end of the series 

 approach very nearly in structure and general appear- 

 ance to the non-parasitic ; at the other end they are 

 extremely different, exhibiting, especially in the 

 males, many most remarkable examples of retrograde 

 development, so that without the study of their 

 metamorphoses it would be quite impossible to re- 

 cognise them as Copepoda, or even as Crustacea of 

 any kind. Yet even in these degraded forms — at any 

 rate in the females- — natatory limbs in a very much 

 atrophied condition are almost constantly found. 



General eorm. — The animal is usually somewhat 

 pear-shaped, rounded in front and tapering towards the 



