V CLASSIFICATION OF MALACOSTRACA I I 3 



fundamentally of a primitive type. We see this especially in 

 the comparative absence of fusion or reduction of the segments 

 of the body externally and of the nervous system internally, and 

 in the simple undifferentiated character of the trunk-limbs, all 

 of which conform to the primitive biramous type. The most 

 anterior thoracic limbs of the Schizopods are of particular 

 interest. In the higher Malacostraca three of these limbs are 

 usually turned forwards towards the mouth to act as maxilli- 

 pedes, and the most anterior of all, the first maxillipede, is apt, 

 especially in the Decapoda, to take on a flattened foliaceous 

 form owing to the expansion of the basal segments to act as 

 gnathobases (see Fig. 1, A, p. 10). Now this appendage in the 

 Schizopods preserves its typical biramous character, and resembles 

 the succeeding thoracic limbs, but in many of the species the basal 

 joints show a tendency to be produced into biting blades (Fig. 

 1, E, p. 10), thus indicating the first step in the evolution of 

 the foliaceous first maxillipede of the Decapoda. The primitive 

 character of the Schizopods is also indicated by the fact that 

 most of the Decapoda with uniramous limbs on the five hinder 

 thoracic segments pass through what is known as the " Mysis 

 stage '' in development, when these limbs are biramous, the exo- 

 podites being subsequently lost in most cases. 



The " Schizopoda " include a very large number of pelagic 

 Crustacea of moderate size, which superficially appear to resemble 

 one another very closely. The slender, elongated body, the 

 presence of biramous limbs on all the thoracic and abdominal 

 segments, and the possession of a single row of gills at the bases 

 of the thoracic limbs, are, generally speaking, typical of the 

 families Mysidae, Lophogastridae, Eucopiidae, and Euphausiidae, 

 which go to make up the old Order Schizopoda. 



It has, however, been pointed out first by Boas,^ and sub- 

 sequently by Hansen and Caiman,^ that the Euphausiidae are 

 in many respects distinct from the other three families, and 

 agree with the Decapoda, while the Eucopiidae, Lophogastridae, 

 and Mysidae agree with the Cumacea, Isopoda, and Amphipoda. 



It has, therefore, been suggested by these authors that the 

 ■classification of the Malacostraca should be revised, and Caiman 

 {loc. cit.) has brought forward the following scheme : — 



' Morphol. Jahrh. viii., 1883, p. 485. 

 2 Ann. Mag. Nat. Hist. (7), xiii., 1904, p. 144. 

 vol,. IV I 



