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joints of tlie maxillae; however, I may also observe that I have found very considerable 

 difference in the length of the olfactory seta of the antennulae between larvae of the same 

 species (Splicer, modesta). Most other larvae we know also differ from each other in a 

 number of features : length of the olfactory seta of the antennulae, structure of the antenn;Te, 

 the relative size of the abdominal segments and of the caudal stylets, and the length of their 

 long setae, sometimes [Mygidion] also In the presence or absence of fine processes on the 

 terminal joint of the maxillipeds. However, in one case, namely in the genus Chonio- 

 stoma, I have not been able to find any difference between the larvae of the two closely 

 allied, yet distinctly separate species. — The size of the ovisacs and the size of their eggs 

 compared with the female present considerable differences between the species, yet they do 

 not natm-ally form good distinguisliing marks. Finally, the few pupae known to me differ 

 very much according to species, except those belonging to the group of Splbcer. Leuckartii. 



b. Limitation and Characters of the Genera. 



All the species may be classed under six genera, which offer an almost regular 

 gradation in tlie reduction of the females. The males too become considerably degraded, 

 but not to such a degree as the females. This gradual reduction is combined with great 

 changes in the way of laying the eggs, whereas there are very little differences and no 

 reduction at all in the structure of the larvae, and the post-larval development is too little 

 known to allow of making general statements about it. Consequently, the arrangement of 

 the genera in the systematic part is easily and naturally carried out in considering the 

 gradual reduction indicated. 



The first genus, Stenothocheres, deviates from all the following by possessing a distinctly 

 prominent abdomen, by the more conspicuous and distinctly two-branched trunk-legs of the 

 females, and by the way they lay their eggs: in one or two (rarely three) free lumps of 

 indefinite form — not in ovisacs, where the eggs are surrounded by a distinct common mem- 

 brane. In the two first mentioned characters the genus approaches the less reduced forms 

 of Copepoda. In the other five genei'a there is no abdomen, the trunk-legs of the females 

 are small with at most one distinct branch, generally consisting of one single joint, or they 

 are altogether wanting, and the eggs are deposed in several or in numerous ovisacs. 



These five genera are naturally divided into two gioups: in the three first, viz. 

 Homoeoscelis, SphceroneUa and Choniostoma, the genital apertures of the females are situated 

 close together and surrounded by a more solid plate, ring or semicircular list, inside which 

 are also found the entrances of the two receptacula seminis; the ovisacs, when laid, are 

 free, not attached to the female. In the other group: Mysidion and Aspidoecia, the genital 

 apertures of the female are often placed at a greater distance or very far from each other ; each 

 has its crescent or ring, and far in front of them is situated one receptaculum seminis; the 

 ovisacs are hinged on the lips of the genital apertures. In the first group most females 



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