THE TANAIDACEA 



191 



and wliicli he placed between Isopoda and Cumacea. Hansen 

 adopts the same view of their affinities, and suggests for the order 

 the name which is here used. Claus's monograph on Afseudea is 

 the most important source of information on the anatomy of the 

 group, and the works of G. 0. Sars are no less important as 

 regards the description and classification of genera and species. 



Morphology. 



The general form of the body (Fig. 116) resembles that ty])ical 

 of the Isopoda, being depressed or sub-cylindrical, with a compara- 

 tively short abdominal region and with the telson coalesced Avith 

 the last somite. The homology of the reduced carapace with that 

 of the Cumacea and Mysidacea is indicated by the small cavity 

 which it overhangs on each side and within which lies the epipodite 

 of. the first thoracic appendage. Only the first and second thoracic 



SC- 



Ai^semlis spinosus, female, ex, vestijjial pxopodites of second and third thoracic limbs ; oc, 

 ocular lobe or peduncle ; sc, scale (exopodite) of antenna ; ur, flagelliform uropod. (After Sars.) 



somites are coalesced with the carapace, but in SjjJiyrapus the third 

 somite is firmly attached to the head-region, although, according 

 to Sars, it is not completely fused with it. 



The eyes are often absent, but when present they are usually 

 set on fronto-lateral processes of the head (Fig. 116, oc), which in 

 many species are defined by grooves and, although not movable, 

 appear to correspond to the ocular peduncles of the podoplithalmate 

 groups. 



Appendages. — The anfennuJes are biramous in the Apseudidae. 

 In some species the two flagella are stated to arise from a common 

 basal segment, so that the peduncle consists of four segments. The 

 antennae have, in the Apseudidae, a small exopodite (Fig. 116, .sc), 

 and the protopodite consists of two segments, not, as is usual in 

 the Peracarida, of three. The mandible carries a palp in the Ap- 

 seudidae, but not in the Tanaidae. The mazilhdae, in the former 

 family (Fig. 117, B), have two endites and a palp of two segments, 

 but in the Tanaidae the proximal endite is wanting and the palp 

 is unsegmented. The maxillae of the Apseudidae (Fig. 117, C) 



