REPORT ON THE ISOPODA. 167 



particularly noticeable. Among the species enumerated above, Arcturus spinosus, 

 Arcturus glacialis, Arcturus purpureus, and Arcturus brunneus are clad with extremely 

 long spines, while the same development is found, though in a less degree, in Arcturus 

 anna and Arcturus spinifrons ; only three deep-sea species out of the ten dredged by 

 the Challenger are unprovided with spines. 



The family Munnopsidse are quite as characteristically a part of the deep-sea fauna as 

 the genus Arcturus, but they do not exhibit this peculiarity to anything like so marked 

 a degree. There is only one species which is distinguished by the presence of numerous 

 spiny outgrowths of the integument, viz., Eurycope spinosa, though in many others, 

 such as the genus Acanihocope, the species Eurycope fragilis and Eurycope atlantica, 

 the epimera and a single row of spines along the back are unusually developed. 



In my account of Serolis I have referred to the length and spiny characters of the 

 epimera in three out of the four deep-sea species, which contrasts very markedly with 

 the flattened, sickle-shaped epimera of those species which inhabit the shore. 



The characters of the deep-sea Asellidas bear out the general truth of what has been 

 said above respecting the Isopodan fauna of the deep sea. The genera Acanthoniscus, 

 AcantJwmunna. and Iolanthe are quite as remarkable for the development of spines as 

 many of the deep-sea Arcturi. 



This modification of structure is not, however, confined to the deep-sea species ; it is 

 found in quite as marked a degree in Arcturus furcatus and Arcturus studeri and in 

 other species of the genus Arcturus ; it will be noticed, however, that these species are 

 inhabitants of the colder regions, and, indeed, it would appear that there is some 

 connection between temperature and the development of spines upon the body. It is 

 more or less true in other groups of the Crustacea that the very spiny forms are either 

 deep-sea, or, if shallow- water, are from the Arctic or Antarctic shores, where the conditions 

 of temperature are not so widely different. 



