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 Mysidion commune. 



I 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 I 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, tliis 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 fiirty-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 Homoeoscelis), live in the branchial 

 cavity of two species belonging respectively to the genera Diastylis Say and Iphinoe 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 

 mi the eye-stalks) of the species of the genus Erythrops G. O. Sars, which belongs to Mysidse 

 verae. 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 speries (the genus Mysidion) on the genera Erythrops G. 0. Sars and Parerythrops 

 Or. 0. Sars, belonging to Mysidse verae, and the tlnee species on which they are found live 

 according to (A. 0. Sars — in a depth varying from 30 to 300 fathoms. An examina- 



