63 



month. The pupa appears to have been attached by a dorsal thread, as in tlie preceding 

 stage, however, it is too badly preserved to allow of a more precise definition. I am 

 nnable to give any more details about tliis stage; I do not see at all how it can be an 

 earlier stage in the development (»t' the female, and coiise(iuently 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 Mijsiilion commune. 



I have now commnnicated in detail all I know about the post-larval development of 

 tlie forms of tliis 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 ditferent species. It 

 would indeed repa}- tlie trouble to cai-ry 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 Avell 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 Choniostomn) live in the 

 branchial cavity of two species of the genus Hippoh/fc Leach, which belongs to the tribe 

 Caridea of the order Uecajioda; tAvo species (the genus HomocosceJis), live in the branchial 

 cavity of two species belonging respectively to the genera Diadylis Say and Iphinoe Sp. 

 Bate, which two genera belong to widely dirtering 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 Ert/throjis G. O. Sars, which belongs to Mysidae 

 verse. All the remainder — thirty-eight species — live in the mai'snpium of species 

 belonging to the following orders: Mysidacea, Cumacea, Isopoda and Ampliipoda; however, 

 their distribution within tliese orders is lather interesting. In Mysidacea I have only found 

 two species (tiie genus Mysidion) on the genera En/throps G. 0. Sars and Parenjthrops 

 G. O. Sars, belonging to Mysidae verae, and the three species on which they are found live 

 — according to G. O. Sars — in a depth varying from oO to ;5()U fathoms. An examuia- 



