234 



anal segment obtusely produced behind. Basal expansion of antennulse divided 

 behind into 6 coarse teeth. Coxal plates very distinctly pectinate. Outer ramus 

 of uropoda extremely small, scarcely half as long as the inner. Colour whitish. 

 Length of adult female 2.10 mm., of male 0.90 mm. Parasitic on Cypridina 

 norvegica, occupying the part of the shell-cavity otherwise destined to receive the 

 ova and embryos of the host. 



Remarks. - As above mentioned, this peculiar parasite was first described 

 by the present author in his ,,0versigt" under the name of Cryptothiria cypridince. 

 The description was only a preliminary one, and, after now having subjected 

 this form to a closer investigation, I am in a position to complete this description, 

 and at the same time to correct an essential error. The fusiform little creatures 

 generally found associated with the adult female, firmly anchored to the Cypridina 

 at one or both sides of the former, are not, as formerly opined, males, but 

 likewise females, though in a very early stage tf development, having lately 

 undergone their transformation. The successive stages figured on PL 97 and 

 marked 9 1 to 9 6 show this fully. Shortly before the transformation the female 

 larva burrows its head deeply into the skin of the host (see figs. 9S 9 2 )? and 

 from its anterior part the long, root-like filaments characteristic of the suc- 

 ceeding stages gradually grow out. Moreover, a highly chitinized reflexed plate, 

 terminating in several strong denticles, developes from the extremity of the head, 

 whereby the attachment becomes still firmer (see the figure at the bottom of 

 the plate on left side). At the same time the fusiform body of the 1st post- 

 larval stage may be distinctly traced within the larval skin, gradually detracting 

 itself from the inner walls of the latter, so as at last only to occupy a part of 

 its inner cavity (see fig. 9 2 )- The empty larval skin is thereupon shed gradu- 

 ally from behind forwards. In the earliest postlarval stage (see fig. 9 3 ) some 

 of the anterior segments of the larval skin are generally seen still loosely 

 covering the neck of the young female, with their appendages in a more or less 

 complete state; but very soon they too are thrown off. and only the deeply 

 immerged cephalic segment of the larva remains, constituting the means by 

 which the female is attached to its host (see fig. 9 4 )- The neck of the latter 

 now becomes gradually more and more constricted, and at last is converted into 

 the narrow flexible string by which the female adheres to the host (see figs. 9 5 > 9 6 )- 

 The form of the body still looks rather different from that of the adult female; 

 but all intermediate stages may be easily demonstrated by the examination of a 

 sufficiently rich material. In the interior of the young female a large bag-shaped 

 body, filled with yellowish contents, may be easily traced. This body, which appears 

 wholly closed in front, and behind extends into the posterior, narrowly exserted 



