217 



obvious, and it must accordingly be assumed, that Rathke, in describing the 

 animal under a new name, did not know the description given by Kroyer. Of 

 course the speeific name proposed by the latter author is that which should be 

 retained lor the species. It is an easily recognizable form, though rather vari- 

 able in shape, according to the degree of distention of the marsupium. m Also the 

 male is somewhat variable both as to size and in the shape of the metasome. 

 As is generally the case with parasitic Isopoda, the immature female exhibits 

 an appearance very different from that of the fully grown animal. In order to 

 show this difference and the successive transformations, several stages are figured 

 on PI. 91, and also figures of the embryo in its last stage, and of the larva in 

 the so-called cryptoniscian stage, are given on this plate. 



Occurrence. — This is by far the commonest of our indigenous Bopy- 

 rids, being met with rather frequently along the whole coast of Norway, from the 

 Christiania Fjord to Vads0, infesting rather a large number of different Crusta- 

 ceans. I have found it on the following 6 species of the genus Spirontocaris 

 (Hippolyte): S. Gaimardii, polaris, pusiola, turgida, spinus. securifrons, and more- 

 over on 3 species of the genus Pandalus, viz., P. Montagui, borealis, and pro- 

 pinqvus. It is also recorded by Dr. Hansen from Spirontocaris Phippsii and 

 by Sp. Bate from S. Barleei, so that at present it is known to occur on no less 

 than 11 different hosts. In all of them, the parasite occupies the very same place, 

 viz.. beneath the anterior part of the metasome, more or less enclosed be- 

 tween the epimeral plates of the first 2 segments (see the uppermost figure in 

 PI. 90). On a closer examination, the parasite is always found to be firmly 

 attached by the aid of the one series of legs to the basal part of one of the 

 anterior pleopoda of its host, sometimes the right, sometimes the left and 

 the distortion of the body to the one or the other side depends on this mode 

 of attachment. The male is found clinging to the ventral face of the metasome 

 of the female, often occupying an oblique position and more or less concealed 

 by the protruding marsupium. In immature females the attached male is often 

 found to be still in the cryptoniscian stage, and I have also occasionally found 

 larval males of this description in different places on fully grown females. The 

 number of eggs and embryos contained in the marsupial cavity is immense, 

 and may amount to several thousand in all. 



Distribution. — British Isles (Sp. Bate), Kattegat (Meinert), Spitsbergen 

 (Norw. North Atl. Exp.), the Kara Sea (Bansen), Greenland (Kr0yer), Grinnels 

 Land (Miers), Atlantic coast of North America (Harger). 



2S. Crustacea 



