89 



the proximal joint broadly rounded and carrying 3 setae, the middle one rather 

 slender, the other 2 very small, distal joint moderately elongated, sublinear in 

 form, with 5 slender setae, 3 of which issue from the tip, 2 from the outer edge 

 close to the end. Ovisac oval in form and generally very large, containing 

 numerous ova or embryos, its colour varying, according to the development, from 

 dark green to light yellowish red. 



Male much smaller than female and of more slender form. Anterior an- 

 tennse more strongly built and distinctly geniculate. Last pair of legs of smaller 

 size than in female, one of the apical setae of the distal joint transformed to a 

 strong ciliated spine. 



Body in female of a whitish colour, and more or less distinctly banded 

 transversally with a clear crimson ; ovarial tubes generally of a very dark colour. 



Average length of adult female about 1 mm., of male 0.65 mm.; maximum 

 length of deep-water variety 1.50 mm. 



Remarks. The specific name furcata assigned to this form by Baird, 

 refers to the strong development of the sensory filament issuing from the 4th 

 joint of the anterior antennae, which gives these organs the appearance of being 

 bifurcate at the end. This is, however, a feature common to all the species of 

 this genus. Whether the Idya barbigera of Philippi in reality belongs to this spe- 

 cies or to some nearly-allied form, it is impossible at present to decide with per- 

 fect certainty; but as the present species is by far the commonest, there is good 

 reason for believing the two to be identical. In any case, however, the specific 

 name proposed by Baird must be retained as the older one. Besides the usual 

 littoral form, another form of considerably larger size is occasionally met with 

 iu somewhat greater depths among decaying algae. I have carefully compared 

 this form with the usual one, but have failed to detect any perceptible anatomical 

 difference. 



Occurrence. This is perhaps the commonest and most widely distributed 

 of all our Harpacticoida, being found everywhere along the Norwegian coast, and 

 generally in great numbers, close to the shore among algse. It is also very often 

 left in tidal pools together with other littoral forms. It is a very active little 

 creature, swimming about with great speed, now and then affixing itself to the 

 fronds of the algae or to the walls of the vessel in which it is being observed. 



Distribution. Arctic Ocean, widely distributed, British Isles, Kattegat, 

 coast of France, Mediterranean, the Red Sea (A. Scott), New Zealand (Brady), 

 Chatham Islands (the present author). 



12 Crustacea. 



