INTRODUCTION. 



that without the evidence derived from the study of 

 their embryology, and the successive phases of their 

 life history, it would be impossible to assign them 

 with any certainty to the place which they are now 

 held rightly to occupy in the scheme of nature. And 

 though none of the species which form the subject of 

 the present Monograph, differ so widely as this, one from 

 another, there yet exist variations of great interest 

 pointing to very decided differences in mode of life. 

 Next to the mouth organs, the anterior antennae and 

 the first and last pairs of swimming feet exhibit the 

 most important variations of structure, the distinctive 

 peculiarities of these parts being mostly connected 

 with the sexual function, and therefore most fully 

 developed in the male sex. The anterior antenna, 

 which in the female is usually a simply-jointed, 

 slender, tapering limb, in the male not unfrequently 

 becomes partially swollen, nodose, hinged, or provided 

 with serrated plates for the purpose of affording a 

 more efficient grasp of the female, these modifications 

 occurring sometimes in one antenna only, as in many 

 Calanidas, sometimes in both, as in Cyclopidse. The 

 modifications of the fifth pair of feet are very various ; 

 in some of the Calanidse they become very powerful 

 auxiliary clasping organs, but more generally they are 

 quite rudimentary in both sexes (Cyclopidas), while in 

 most Harpacticida3, though rudimentary in the male, 

 they are usually somewhat more largely developed in 

 the female, where they are foliaceous in character and 

 appear to act as a support or protection to the 

 external ovisac. 



