158 M. Steenstrup on the genera Pachybdella and Peltogaster. 



I am not aware that Professor Kroyer has since published 

 anything upon this subject ; but from the preceding it is evident, 

 that at least the two first-mentioned vermiform species are true 

 Peltogastri, and that the third must also be placed very near 

 this genus, notwithstanding the difference in the form of the 

 body, appears from the fact that Kroyer places them all in 

 one and the same genus. If the interesting observation upon 

 the young given by Kroyer in the preceding note applies to all 

 the three species, it becomes a positive observation of the 

 Crustacean nature of the vermiform Peltogastri ; but if it 

 applies only to the third and last form — to which, as is evident 

 from the text, the note in which the observation is given par- 

 ticularly belongs, — it is at any rate a new confirmation of the 

 opinion that these sac-like structures, filled with eggs, which 

 occur under the abdomen of the long- tailed crabs, are them- 

 selves to be regarded as Crustacea. 



Even if it may remain doubtful to w^hich of the smaller natural 

 divisions of the Crustacea the genera in question are to be re- 

 ferred, the above-mentioned observations upon the form of the 

 young show distinctly that they are Crustaceous animals ; so that 

 it is evident they cannot be represented in our systems as her- 

 maphroditic animal forms. As long as it is not universally 

 admitted that the separation of the sexes, or unisexuality, is a 

 general rule (not to say, law) in nature, and that in our science 

 it must not be admitted that any single animal possesses an 

 opposite sexuality or hermaphroditism, without a scientific proof 

 of this abnormal behaviour with regard to this particular 

 animal, the opponent of hermaphroditism, which is still ascribed 

 to a good many animals, must find himself in this position, — 

 that the sexual relations of the less known and uninvestigated 

 animal forms are adopted from the relations of those animals 

 which are most nearly allied to them, and which have been sub- 

 mitted to a closer examination in this respect, although innu- 

 merable examples of the uncertainty of this procedure are suffi- 

 ciently well known. Thus, as long as the two genera of parasites 

 under consideration could be regarded as Hirudinoid animals, 

 we were under the necessity of supposing them to be herma- 

 phrodites, although this point was not only not proved, but had 

 never even been investigated, but because all the Hirudinese were 

 regarded as hermaphrodites. But if we now know that these 

 animals are Crustacea, and agree that all the Crustacea — with 

 the exception of most of the forms belonging to the group of 

 Cirripedia, to which they can scarcely be referred — are to be 

 regarded as animals with separate sexes, these parasites must 

 also be considered from analogy as unisexual animals. To show 

 this, and to prove that it was far from right to seek in these 



