Vv CLASSIFICATION OF MALACOSTRACA 113 
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 lmbs, 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 Calman,” 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 Calman 
(Joc. cit.) has brought forward the following scheme :— 
1 Morphol. Jahrb. vili., 1883, p. 485. . 
2 Ann. Mag. Nat. Hist. (7), xili., 1904, p. 144. 
VOL. IV I 
