INTRODUCTION. 



Of all the groups of the Copepoda, that of the Harpacticoida is undoubt- 

 edly the most extensive, and at the same time the most difficult to examine, 

 on account of the generally very small size of the forms comprised within it, 

 some of them being indeed almost undistinguishable to the naked eye. The most 

 prominent character whereby the present group is distinguished from that of the 

 Calanoida is, as clearly shown by Dr. Giesbrecht, the very movable articulation of 

 the last segment of the metasome with the preceding segment, and its firm con- 

 nection with the 1st segment of the urosome, giving it the appearance of more 

 properly belonging to that section of the body. Thus far this group perfectly 

 agrees with that of the Cydopoida, and. indeed Dr. Giesbrecht comprised both in 

 his division Podoplea. I find it, however, more appropriate to keep these 2 groups 

 apart, as in other respects they differ from each other rather materially. By 

 most authors, the group here in question is regarded only as a family, Har- 

 pacticidce; and the numerous genera comprised within it have of course been ar- 

 ranged under a series of subfamilies. In accordance with the arrangement adopted 

 by the present author, as regards the Calanoida, these subfamilies are here re- 

 corded as true families, and their number has been considerably augmented. 

 Moreover an attempt has been done to group these families under a few sections, 

 as will be shown farther on. 



The type of the present group is the genus from which its name is 

 derived, viz., Hqrpactifti* Milne-Edwards; and indeed this genus seems to combine 

 some of the most characteristic features of the group, and in all probability con- 

 stitutes a very primitive form. The difference, both as regards the external appear- 

 ance and the structural details, between this genus and those of the Calanoid and 

 Cyclopoid groups is very conspicuous ; yet we find many deviations from this 

 type, tending partly towards that of the Calanoida, partly towards that of the 

 Cydopoida, and in most cases a very gradual change in one or other direction 

 may be easily demonstrated. 



1 Crustacea. 



