424 Dr. H. J. Hansen on the Morphology of the 



Nusbaum), which must undouhtedly he homologous lolth the 

 furcain Nebalia; they are relatively of large size, especially 

 in the penultimate larval stage, and are probably cast off in 

 the penultimate ecdysis in the marsupium. 



19. The Cumacea, Tanaida3 (which must be elevated into 

 a separate order), Isopoda, and Amphipoda agree exactly in 

 the structure of the mouth-parts (not including the suppression 

 of the exopodite of the maxillai, the suppression of the 

 lacinia mobilis in isolated parasitic forms, and similar 

 secondary reductions) and thoracic appendages with the 

 Mysida3 in all the characters which are mentioned in § 18, 

 It is very easy to study the structure of the maxillffi and 

 maxillulge in Amphipoda {vide " Dijmphna-Togtet ") and 

 Isopoda {vide " Cirolanidas'"). In certain Isopods the stem of 

 the antennffi is distinctly three-jointed, but in the majoriry of 

 forms the first segment disappears and probably fuses with 

 the head. Even where the so-called epimeres are strongly 

 developed (as in certain Amphipods) the coxopodite (or the 

 interval between the body and the articulation of the second 

 segment) is always short and several times shorter than the 

 basipodite. 



20. In many Isopods the first segment of the six hindmost 

 pairs of thoracic limbs is small and movable ; in many other 

 genera it develops as an " epimere," which, e. g. in Idothea 

 entomon, is very large, marked off on the dorsal side by an 

 arthrodial furrow, and on the ventral surface fused with the 

 ventral ridge (" Bauchschiene ") ; hnt in Idothea hectica the 

 upper arthrodial furrow ]]as also disappeared and the " epi- 

 mere " has certainly not been reduced to the point of disap- 

 pearance, but has become fused with the segments of the 

 body and forms larger or smaller portions of their sides 

 (c/;§§24and49). 



21. In certain Isopods {Jamra) we find a large thorn upon 

 the seventh segment of the thoracic appendages, articulated 

 beneath the cheliform eighth segment, so that the foot is 

 said to have two claws {cf. § 51). 



22. The Euphausiidffi are a long way removed from the 

 Mysidffi in consequence of a series of characters, certain of 

 which only will be mentioned here. The stem of the antennje 

 is two-jointed ; the mandibles have no lacinia mobilis. Tlie 

 limbs are composed of only seven segments (they are without 

 the " claw "). The " knee " lies between the fourth and fifth 

 segments ; the first segment is almost equally as large as 

 the second. I suppose the segments beyond the knee to be 

 homologous with the sixth, seventli, and eighth (claw) 

 segments in Mysis^ and the fourth segment to be homologous 



