172 THE VOYAGE OF H.M.S. CHALLENGER. 



The uropoda (fig. 31) are still rather small, but have their basal part and terminal 

 plates well defined, the latter exhibiting a few slender marginal bristles. The outer 

 plates arc highly distinguished by the remarkable length of the spine jutting out from 

 the outer corner ; moreover, this spine is finely denticulate along the inner edge. 



Habitat. — The larva described above was taken December 24, 1873, in the Southern 

 Ocean, off Prince Edward Island. 



Family 4. Mysid.e. 



This family, the lowest in rank among the Schizopoda, comprises both littoral and 

 pelagic forms, as also true deep-sea animals. In none of them is the slightest trace of 

 true gills to be observed, and they are thereby very sharply defined from the three 

 preceding families of Schizopoda, in which the gills are invariably well developed. In 

 some forms, however, a peculiar folding of the integument, covered by the free parts of 

 the carapace, can be discerned, and this structure may possibly stand in some relation 

 to the respiratory function, though scarcely corresponding morphologically to the true 

 gills in other Podoplithalmia. I first called attention to this peculiar structure as 

 early as the year 1867, when describing the fresh- water variety of Mysis ocidata, 

 Fabr. {Mysis relicta, Loven),^ and shall in the present Eeport describe a similar 

 structure in the large deep-sea Mysidan, Boreomysis scyphoiys, G. 0. Sars. Another 

 appendage, peculiar to the males only, and issuing from the base of the inner branch 

 of the pleopoda, may perhaps be also regarded as subservient to respiration. More 

 especially in the males of the genus Siriella, Dana (Cynthia, Thompson), do these 

 appendages present an appearance that strongly recalls that of true gills. 



As a character common alike to all Mysidans, and sharply distinguishing them 

 from other Schizopoda, may be mentioned the rudimentary state of the caudal limbs in 

 the females, forming, as they do, very small setifei'ous lamellse that have no relation 

 whatever to locomotion, and thus have little or no claim to the term " pleopoda." 

 This, in some genera, as Mysis, Heteromysis, Mysidella, also applies to the males. 

 But in most of the genera the caudal limbs in the males are modified so as to assume 

 the character of true natatory organs, being constructed in a manner similar to the 

 pleopoda in the Lophogastridse and Eucopiidte. 



The Mysidse comprise numerous genera, most of which are met with in the Northern 

 Ocean, and some of the species, as Mysis oculata, Fabr., are at times found crowded 

 together in enormous shoals, thus serving as food for whales and other large vertebrates. 



The Challenger collection comprises fifteen species of Mysidse, belonging to eight 

 g.?nera, one of which is new. 



' Histoire naturcUe des Crustaces d'eau douce de Norv^ge, pt. i. 



