a new Parasitic Isopod Crustacean. 91 



margin, only three joints can be distinguished, namely, a cylin- 

 drical basal joint, a minute second, and an elliptical terminal 

 joint, the latter 0*04 millim. in length, and half that breadth. 

 This pair of feet is usually applied close to the body, with the 

 basal joint turned inwards, and the terminal joint backwards. 



The abdomen bears, first of all, four pairs of natatory feet 

 with a crescentiform basal joint, which is attached nearly by the 

 middle of the convex side in such a manner that one horn, which 

 is a little the shorter, is directed inwards and somewhat back- 

 wards, and the other forwards and outwards. The distance be- 

 tween the horns is 0*03 millim. The outer horn bears a lancet- 

 shaped terminal joint, which fits exactly into the emargination 

 of the crescent, and bears at its obliquely cut-off extremity three 

 bristles of about twice the length of the joint. Sometimes this 

 terminal joint is distinctly smaller on the fourth pair than on 

 the three preceding ones ; but usually they are all alike. On the 

 inner horn of the three anterior basal joints there is a single 

 bristle; sometimes I found all these bristles of equal length, 

 rather longer than those of the terminal joint; but more fre- 

 quently the second and third perceptibly shorter, and the last 

 only one-third of the length of the first. The fifth abdominal 

 segment bears a narrow and short appendage, without any bris- 

 tles (fig. 15), which is cleft into a longer interior and a shorter 

 exterior point. Lastly, at the sides of the last abdominal seg- 

 ment, there are appendages of considerable size, with a thick 

 basal joint and two slender biarticulate terminal branches, of 

 which the outer is shghtly the longer. The last, spiniform joint 

 of this branch is straight ; a shorter spine occurs externally at 

 the apex of the basal joint and of the first joint of the branch. 



The little animals do not creep with much activity, but swim 

 very rapidly. Their dorsal natation, combined with the long 

 bristles of the swimming-feet and the forked appearance of the 

 tail, owing to the lateral appendages, gives them a distant re- 

 semblance to Cyclops. 



The female of Entoniscus is so placed in the interior of the 

 Porcellana that its head lies concealed amongst the cseca of the 

 liver ; it then passes backwards and beneath the heart to the 

 extremity of the cephalothorax, the brood-leaves even reaching 

 sometimes pretty far into the abdomen. The entire animal, 

 including even the head and mouth, is rather closely embraced 

 by a membranous sac, which is continued posteriorly into a 

 narrow efferent canal, and may be traced as far as the suture, 

 between the sternum and the free segment, which, in the For- 

 cellance, bears the rudimentary fifth pair of feet. This envelop- 

 ing sac is probably produced by the young Entoniscus, in its 

 passage into the interior of the Porcellanaj not breaking through 



