454 CRUSTACEA MALACOSTRACA. 



tinction in this respect between the Decapod Sergestes and the 

 Schizopod Gnathophausia. Great variation in the degree of 

 fusion of the thoracic segments to form a cephalo-thorax is found 

 in Schizopods, and, as Caiman points out, the fusion is as complete 

 in Euphausia as in any Decapod. 



Biramous thoracic legs are widely found among the Mala- 

 costraca, and in the lower Decapods as elsewhere. The presence 

 of a single series of subdivided branchiae in the Schizopods, 

 against a possible four to each thoracic appendage in the Deca- 

 ppda appears a more substantial character, but it is doubtful 

 whether the arthrodial branchiae of the Lophogastridae are 

 precisely homologous with the podobranchial appendages of 

 the Hemitropha. 



On the other hand it has been pointed out that though 

 the Schizopoda agree in a general " caridoid facies " their 

 two divisions, Hemitropha (Euphausiidae) and Holotropha 

 (Mysidae and Lophogastridae) are strongly contrasted in many 

 of the characters in which the Decapoda are distinguished from 

 other divisions of the Malacostraca. Thus the Hemitropha 

 agree with Decapods in the absence of a brood-pouch, and, in 

 association with this feature, in the presence of free larval stages 

 in the development ; in the participation of all segments of the 

 thorax in the formation of the cephalothoracic shield ; in the 

 short capsular heart ; the absence in the adult of a lacinia mobilis 

 on the mandible (see p. 438) (though there are indications of it in 

 some larval stages) and in the spherical or vesicular shape of 

 the spermatozoa. 



The Holotropha, on the other hand, agree with the Cumacea, 

 Chelifera, Isopods and Amphipods in the possession of a brood 

 pouch, in which the young are developed until the full number 

 of appendages is attained. The terga of only the most anterior 

 (or none ? Lophogastridae), of the thoracic segments are involved 

 in the dorsal shield, the heart is elongated, a lacinia mobilis is 

 present, and the spermatozoa, so far as they have been observed, 

 are filiform. The two groups also appear to be contrasted 

 in the number of segments of the thoracic limbs, the point at 

 which the main flexure of the limb occurs and in the possession 

 by the limb of a terminal claw. 



In reviewing these differences, to several of which attention 

 has been called by Boas and Hansen, Caiman has proposed the 



