820 A HISTORY OF RECENT CRUSTACEA 



flattened spines being of use for specific distinctions. One 

 or both of the pairs have usually small two-jointed exopods. 

 The perjeopods are slender, with many setas and spines. 

 The pleopods have two long setiferous branches, and the 

 uropods two that are many-jointed and filiform, the inner 

 elongate. 



The small exopods above mentioned, when seen only 

 in cabinet specimens, might be considered rudimentary or 

 of little importance. But this would not be a right way 

 of regarding them. The branchial chamber has two little 

 openings, one for the entrance, the other for the exit of 

 the water used in breathing. In front of the latter of 

 these orifices in the living animal, M. Yves Delage points 

 out that the exopods of the first gnathopods with their 

 terminal setas vibrate so rapidly that the machinery can 

 with difficulty be seen, while those of the second gnatho- 

 pods behave in the same manner in front of the entrance 

 apertures. By this contrivance a lively current of water 

 is introduced to the branchial chamber, and, when its 

 virtue has been exhausted, the parting guest is speeded to 

 a convenient distance. When there is only one pair of 

 exopods, it is those at the gate of egress which remain, 

 and it is no doubt of less importance that the entering 

 current should be rapid than that the used-up water 

 should be effectively dismissed. 



Five genera are at present assigned to this family. 

 The ocular lobes are not movable, as they were at one 

 time supposed to be. 



Apseudes, Leach, 1814, has six segments of the trunk 

 free, a well-developed scale on the second antennae, a well- 

 developed ' palp ' on the mandibles, exopods on both pairs 

 of gnathopods, and five pairs of pleopods, with both 

 branches usually one-jointed. 



Parwpseudes, Sars, 1880, has six segments of the trunk 

 free, the antennal scale rudimentary, the mandibular 

 'palp' very small, exopods on both pairs of gnathopods, 

 and only four pairs of pleopods, which have one of the 

 branches two-jointed. The single species is Parap/teudes 

 laiifrons (Grube) from the Mediterranean. It was at one 



