156 Dr. A. Metzger on the Mais and Female of LernEea. 



Third and fourth pairs of natatory feet imiramose, in other 

 respects agreeing with the preceding. 



The entire little animal, which is scarcely more than f line 

 in length, is translucent, and of a peculiar bluish-grey colour, 

 with the exception of some particular parts of the body, which 

 contain a pigment varying from dark violet to blue. 



The female form is distinguished from the male (1) by the 

 want of the second pair of maxillipedes, and (2) by the elon- 

 gated, nearly uniformly cylindrical and slightly bent abdomen, 

 in which the genital segment and caudal piece are not distin- 

 guishable externally. The two apical processes (furca) are 

 excessively minute, and beset only with two or three short 

 bristles. The whole surface of the abdomen also shows an 

 extremely fine and regular transverse striatum, in consequence 

 of which the margins of the abdomen, when slightly pressed 

 with a glass cover, appear as if denticulated. 



The natatory feet, the first pair of maxillipedes, the buccal 

 cone, and the antenna? do not differ from those of the male ; 

 but whilst in all the male individuals which were found united 

 Avith females the genital segment was swelled, and presented 

 a spherical inflation at each of the two spots where the genital 

 apertures are situated, nothing of the kind, indicating the com- 

 mencement of the business of generation, was to be observed 

 in the females. Even in further advanced individuals, already 

 in course of retrograde metamorphosis, in which the cephalo- 

 thorax and the three free thoracic segments were no longer 

 distinguishable, but which still all possessed the two pairs of 

 antenna?, the pair of maxillipedes, and the four pairs of nata- 

 tory feet, with the basal joint somewhat abbreviated however, 

 and which also showed some of the above-mentioned pigment- 

 spots, no inflation of the abdomen by sexual materials was 

 observable. The abdomen was only a good deal elongated, 

 strongly twisted into a sigmoid form, and even showed still 

 under the thin horny coat the transverse striation so charac- 

 teristic of the female form. Nevertheless I believe that copu- 

 lation takes place in the stage of development above described, 

 in favour of which we have not only the union of the two 

 sexes so frequently observed by me and always taking place 

 in the same manner, but also the circumstance that males have 

 never been found even upon the forms of Lernwa in course of 

 transformation and not yet furnished with egg-threads. After 

 the completion of copulation the female quits the branchial 

 lamina? of its host, and seeks instead of them the branchial 

 arches of the same or of some other fish. It is only here that 

 the horns, which effect a permanent fixation, and which, like 

 the adherent organ of Lerna'ojjoda, represent the second pair 



