Classification of the Crustacea Malacostraca. 149 



membrane, and posteriorly it is firmly connected with the 

 first abdominal somite. It is to all appearance quite compa- 

 rable to the articular surface (e) on the dorsal aspect of the other 

 abdominal somites, which is concealed beneath the posterior 

 margin of the somite in front when the abdomen is extended, 

 and it only differs from these articular surfaces in being more 

 sharply defined from the somite of which it forms a part. 

 It is possible, though I know of no evidence to support the 

 view *, that this plate is actually the tergal portion of the 

 last thoracic somite, which has become detached from the 

 sternal portion and has coalesced with the succeeding somite, 

 but, in any case, the structure is exactly alike in Euphausiidae 

 and in the lower Decapods. I have carefully sought for 

 other evidence of a distinct tergal element of the last thoracic 

 somite in Euphausiidae, but without success, and I can only 

 conclude that the statement of its existence is an error based 

 upon the observation of this intermediate plate without direct 

 comparison with the Decapoda. 



One point in which the Euphausiacea appear to agree 

 with a section of the Mysidacea and to differ from the 

 Decapoda is the possession of a single series of branchiae at 

 the bases of the thoracic limbs. In the Decapoda the gills 

 are arranged in several (typically four) series. Those of the 

 Euphausiacea are attached to the coxopodites of the limbs,, 

 corresponding to the podobranchiae (and epipodites) of the 

 Decapods, from which, however, they differ in their mode of 

 branching. In the Lophogastridae and Eucopiidas, on the 

 other hand, the gills are attached to the articular membrane 

 at the base of the limbs, and are, in fact, arthrobranchise. 

 As Claus has pointed out, this difference in the place of 

 attachment does not necessarily invalidate the comparison 

 between the branchiae of the two groups, since he has shown 

 that in certain Decapods the arthrobranchiae develop as out- 

 growths from the basal portions of the limbs, and that the 

 pleurobranchiae had in all probability a similar origin. There 

 is, however, another fact which may have a bearing on this 

 question. In Gnathophausia (Lophogastridae) Sars describes 

 a small tongue-like process, tipped with a group of seta?, on 

 the outer side of the coxopodite of all the thoracic limbs 

 except the first pair, and he regards this as a reduced epipo- 

 dite. It seems not unlikely that this process, and not the 

 gill itself, is homologous with the epipodial gill of the 



* Williamson figures this plate as a separate sclevite in the larva of 

 Cratigon, " On the Larval Stages of Decapod Crustacea. — The Shrimp 

 (Crangon vulgaris, Fabr.)," Rep. Fishery Board Scotland, xix. (3) 1901, 

 pi. v. rig. lob', " in." 



