As mentioned above, the habits of the Cyclopoida are much more varied 

 than is the case with the Calanoida and Harpactieoida. Some of them, for 

 instance the species of the genus Oithona, are true pelagic animals, constituting 

 a chief part of the plankton in almost all regions of the Oceans. Several other 

 forms, like the Harpactieoida, live near the shores among algae or on a muddy 

 bottom at different depths, and a great number of forms lead a more or less 

 pronounced parasitic existence, attacking different invertebrate animals, in some 

 instances also fishes. In most cases, however, the parasitism may be said to 

 be merely temporary, though there are also a few examples of a more permanent 

 parasitism, for instance in the case of Ergasilus and Nicothoe. The different mode 

 and degree of parasitism has a great influence on the structure of the oral parts, 

 and indeed we have in this feature a good guide for an exact subdivision of this 

 group of Copepoda. The parasitic forms exhibit, as regards the structure of the oral 

 parts, 2 well-marked types, both again differing conspicuously from the non- 

 parasitic forms. We may accordingly divide the Cyclopoida into 3 natural sec- 

 tions, for which I propose to retain the well-known names: gnathostoma, siphono- 

 stoma and poecilostoma. These names were first proposed by Thorell, but were 

 taken in a much wider sense, viz., to divide the entire order Copepoda. I here 

 restrict them to the group of Copepoda which will be treated of in the present 

 volume. 



As to the general characters distinguishing this group, the external 

 appearance of the body, as a rule, looks rather different from that found in the 

 typical Harpactieoida by the very sharp demarcation of the 2 chief divisions of 

 the body, the anterior one being more or less tumefied, the posterior very narrow. 

 So far the Cyclopoida more resemble the Calanoida than the Harpactieoida, though 

 they are easily distinguished from them by the very moveable articulation between 

 the last 2 trunk-segments, the posterior of which, as a rule, is very small and 

 firmly connected with the genital segment, so that, at first sight, it has the 

 appearance of belonging more properly to the posterior than to the anterior 

 division of the body. 



The anterior antennae are, as a rule, more elongated than in the Har- 

 pactieoida and are composed of a greater number of articulations. There are, 

 however, also some instances of a more or less great reduction of these limbs, 

 both in size and in the number of joints. 



The posterior antennae are generally simple, without any outer ramus or 

 exopodite. Only in a few of the parasitic forms is there a slight rudiment of 

 such a ramus. In several of the poecilostomous Cyclopoida these antennae are 

 transformed into powerful prehensile organs. 



