238 



Aseoniscus simplex, G. 0. Sars, n. sp. 

 (PI. xcvm, fig. 3). 



Specific Characters. Body of adult female forming an oval pyriform, soft- 

 skinned pouch hanging down from the empty larval skin, the posterior part of which 

 is generally found to be thrown off; that of young female more fusiform or rather 

 sigmoid in shape, with the posterior extremity conically produced and slightly wrinkled. 

 Male (or female in the last larval stage) oblong oval, tapering both in front and 

 behind, dorsal face finely striolate transversally. Cephalic segment narrowly 

 rounded in front; anal segment almost transversally truncated behind. Eyes 

 wanting. Basal joint of antennulee produced behind to a narrow liiiguiform expansion 

 pointing straight posteriorly, and tipped by 2 strong, unequal setse. Antennae 

 rather elongate, extending to the end of the 5th pedigerous segment. Legs of 

 about same structure as in the male of Cyproniscus. Uropoda with the outer 

 ramus well developed, though shorter than the inner, which tapers to a very nar- 

 row point. Length of adult female 4 ! /2 mm., of male about 2 mm. Parasitic 

 on Boreomysis arctica (Kroyer). 



Remarks. This is the only as yet known species of the genus, and exhi- 



\ 



bits considerable interest on account of {he extreme retrogression of the female, 

 which, strictly speaking, represents nothing more than an ovarian pouch, its con- 

 nexion with the larval individual being only a mechanical, not an organic one. 

 Indeed, the larva from which the female is developed dies off completely, and it 

 is only to the inside of the empty skin of the former that the female is connec- 

 ted. In all the specimens examined, both young and adult, the larval skin was 

 incomplete, a great number of the posterior segments having been thrown off. 

 The occurrence of this form under very similar conditions to those under which 

 the species of the genus Dajus, belonging to a quite different family, are found, 

 clearly shows that the principle of classification adopted by MM. Giard and 

 Bonnier is quite unmaintainable. 



Occurrence. I have found this peculiar parasite occasionally within the 

 incubatory pouch of the deep-water Mysidian Boreomysis arctica, Kroyer, taken 

 partly in the Trondhjem Fjord partly in the Christiania Fjord. In all instances, 

 unlike what is the case with Mysidse infested by Dajus, the marsupial pouch of 

 the host was fully developed, and it thus seems that the parasite must have 

 entered the pouch immediately after the young of the host had escaped. Several 

 male specimens were found associated with the female, and in one instance 2 or 

 3 females of different development were found in the same host. The male, or 

 last larval stage, I have also found occasionally free in the sea. 



