63 
mouth. The pupa appears to have been attached by a dorsal thread, as in the preceding 
stage, however, it is too badly preserved to allow of a more precise definition. I am 
unable to give any more details about this stage; I do not see at all how it can be an 
earlier stage in the development of the female, and consequently be followed by the two 
above-described stages; so it may possibly be a male pupa; however, it must be left to the 
future to solve these and other problems in the remarkable development of MZysidion commune. 
IT have now communicated in detail all I know about the post-larval development of 
the forms of this family. Being unable, on account of the great gaps, to generalize very 
much, I have preferred to collect all L know in this place, instead of contenting myself with 
making a shorter extract and distributing the greater part among the forms in question in 
the later systematic representation. Though I think I have found a series of rather intere- 
sting facts, this is only the beginning of a complete elucidation of the very peculiar meta- 
morphosis of these animals with their extraordinary variations in the different species. It 
would indeed repay the trouble to carry out such an investigation in numerous representatives 
of this family, but it would at the same time present enormous difficulties, on account of 
the nature, as well as of the rarity, of the material. 

B. Habitation, Biology and Distribution. 
a. The Place of the Hosts in the System and the Habitation 
of the Parasites. 
Of the forty-three species examined by me, two (the genus Choniostoma) live in the 
branchial cavity of two species of the genus Hippolyte Leach, which belongs to the tribe 
Caridea of the order Decapoda; two species (the genus Homocoscelis), live in the branchial 
cavity of two species belonging respectively to the genera Diastylis Say and Iphinoé Sp. 
Bate, which two genera belong to widely differing families of the order Cumacea; one species 
(the genus Aspidoecia) lives on the outside of the body (on the carapace, on the back and 
the sides of the last free thoracic segment and of the six first abdominal segments, as well as 
on the eye-stalks) of the species of the genus Hrythrops G. O. Sars, which belongs to Mysidee 
vere. All the remainder — thirty-eight species — live in the marsupium of species 
belonging to the following orders: Mysidacea, Cumacea, Isopoda and Amphipoda; however, 
their distribution within these orders is rather interesting. In Mysidacea I have only found 
two species (the genus Mysidion) on the genera Erythrops G. O. Sars and Parerythrops 
G. O. Sars, belonging to Mysidz verze, and the three species on which they are found live 
— according to G. O. Sars — in a depth varying from 30 to 300 fathoms. An examina- 
