INTRODUCTION. 



Uf all the orders of Crustacea, that of the Copepoda is perhaps the most 

 extensive one, comprising, as it does, an immense number of forms, which, both 

 as to structure and habits, exhibit quite wonderful variation. Some of them 

 lead a free existence as pelagic organisms, and are comparatively highly orga- 

 nized; some are more sedentary in habits, being restricted to the bottom, or are 

 partly semiparasitic; and a considerable number are true parasites, becoming so 

 much degenerated in the adult state, that they sometimes even hardly admit 

 of being recognized as Crustacea, but look more like worms. It is easy to 

 believe that the parasitic forms have originally descended from free-living forms; 

 and indeed, the semiparasitic Copepoda form a well-marked transition between 

 the 2 groups. In consequence of this, the most primitive characters must be 

 sought for not among the parasites, but among the free-living forms. During 

 the development, which is always at true metamorphosis, all the Copepoda 

 pass through some free-living stages, the earliest of which is the well-known, 

 so-called Naiqrtian stage, also found in some other Crustacea, for instance the 

 Phyllopoda and the Euphausiidce. This very primitive stage is characterised by 

 a rounded or oval, unsegmented body, which only carries 3 pairs of movable 

 appendages, viz., the antennulre, the antennae, and the mandibular legs, the mouth 

 being protected in front by a large flap-shaped lip. The segmentation of the 

 body takes place gradually in the succeeding stages, but never attains that 

 elaborate arrangement found in the higher Crustacea (the Malacostraca), and is 

 sometimes again wholly lost in the parasitic life. As is clearly shown by the 

 development of the Euphausiidce, and by the structure of the Pliyllocarida, one 

 of the chief divisions found in the higher Crustacea (Malacostraca), and often 

 constituting by far the greater part of the body, viz., the mesosome, never comes 

 to development in the Copepoda. On the other hand the posterior part of the 

 body is, as a rule, like that in the Pliyllocarida, divided into 2 sharply denned 



1 Crustacea. 



