52 



The first abdominal segment always dilates considerably from its base towards its 

 end, and the free posterior angle has a powerful, often spiniform, seta, the length of which 

 varies between being a little longer than the following segment (Stenothocheres, pi. I, fig. 1 1) 

 and being longer than the whole abdomen, and plumose in its distal half (Choniostoma. 

 pi. XI, fig. 1 e). Inside or outside this seta and close to it there is always another seta, 

 which, as a rule, is much shorter, and only in Sphcer. microcephala (pi. VIII, fig. 2i) is 

 remarkably long, though somewhat shorter than the first one. The second segment is 

 sometimes shorter, and in this case not unfrequently somewhat narrower, than the first one, 

 e.g. in Choniostoma, sometimes quite as long, and always without setae. In Stenothocheres 

 (pi. I, fig. 1 1) the third segment, together with the not separated caudal stylets, forms a large 

 and broad, elongate segment, much larger than any of the preceding ones, and incised 

 posteriorly in the median line. In all other species the third segment, together with the 

 caudal stylets, is nearly always somewhat, and generally much, smaller than the second 

 segment, and the stylets are sometimes not set off from the segment, but most frequently 

 distinctly articulated on it as two short, almost cylindrical joints. Each stylet has always 

 a very long and thick, sometimes plumose, seta, which in Stenothocheres egregius is only a 

 little longer than the abdomen, in Mysidion and Aspidoecia somewhat longer, though not 

 nearly half as long as the cephalothorax , in Homoeoscelis , Sphceronella and Choniostoma 

 longer than half the length of the cephalothorax, and sometimes attaining to three quarters 

 the length of this part, e. g. in Sphcer. dispar (pi. IX, fig. 3 k). Outside this long seta each 

 stylet has in Stenothocheres four, in the other species two or three, comparatively short setae. 



Whereas there were great differences between the females among themselves and 

 between the males among themselves in the different genera and species, we see from the 

 detailed description given above, that all larva I know are surprisingly uniform, so much 

 so, that I have been able to find rather insignificant generic characters only in Stenothocheres 

 and Homoeoscelis, as distinct from the four other genera; at the same time the larvae of 

 Mysidion and Aspidoecia — whose females deviate much from those of the other genera 

 with regard to receptaculum seminis, the position of the genital apertures and the Mngement 

 of the ovisacs — deviate less from various larvae of Sphceronella, than these differ from 

 each other. 



III. Further Development of the Larvae. The larvae, after making their way out 

 of the ovisac, — at least as a rule, swim out and seek a new host. I cannot deny the pos- 

 sibility that one or a few of the larvae may remain in the branchial cavity or in the 

 marsupium of the mother's host, though I doubt it very much. In a marsupium which 

 was infested beforehand I have repeatedly found one or several larvae, which were 

 decidedly invaders. I met with the greatest invasion in a specimen of Hippolyte Gaimardii, 

 where in one of the branchial cavities I discovered a very young female of Choniostoma mirabile 

 and certainly more than fifty larvae and pupae hinged on the gill-fibres (s. the special 

 description below). 



