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joints of the maxille; however, I may also observe that I have found very considerable 
difference in the length of the olfactory seta of the antennule between larve of the same 
species (Spher. modesta). Most other larvee we know also differ from each other in a 
number of features: length of the olfactory seta of the antennule, structure of the antenne, 
the relative size of the abdominal segments and of the caudal stylets, and the length of their 
long sete, sometimes (Mysidion) 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 larve 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 naturally form good distinguishing marks. Finally, the few pupe known to me differ 
very much according to species, except those belonging to the group of Spher. Leuckarti. 
b. Limitation and Characters of the Genera. 
All the species may be classed under six genera, which offer an almost regular 
gradation in the 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 larvee, 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 
eradual reduction indicated. 
The first genus, Sfenothocheres, 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 genera 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 groups: in the three first, viz. 
Homocoscelis, Spheronella 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 Aspidoccia, 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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