CLASS CRUSTACEA. 321 



furnished with pinnulae, or composed of lamellate articula- 

 tions. Their brain is formed but of one or two lobules. The 

 heart has always the form of a long vessel. The gills, com- 

 posed of hairs or setae, either isolated or joined together like 

 beards, combs, or aigrettes, constitute a part of these feet, or 

 of a certain number of them, and sometimes of the mandibles 

 or of the upper jaws. (See Cypris.) Hence is the origin of the 

 word hrcmchiopoda, which we have applied to these animals, 

 of which at first we had formed but a single order. Almost all 

 of them have a tes*ta, or shell, composed of one or two pieces, 

 very thin, and most frequently nearly membranaceous and 

 diaphanous, or at least they have a large anterior thoracic 

 segment, often confounded with the head, and appearing to 

 be a substitute for the testa. The teguments are generally 

 rather corneous than calcareous ; which approximates these 

 animals to the insects and arachnida. In those which are pro- 

 vided with the usual jaws, the lower or exterior ones are always 

 uncovered, all the jaw-feet performing the office of feet pro- 

 perly so called, and none of them being attached to the mouth. 

 The second jaws, those of the phyllopoda, at most, excepted, 

 even resemble these latter organs. Jurine has sometimes de- 

 signated them mider the name of hands. 



Such are the characters which distinguish the grinding or 

 masticating entomostraca from the malacostraca ; the other 

 entomostraca, those which compose our order of ptecilopoda, 

 cannot be confounded w'ith the malacostraca, because they are 

 destitute of organs adapted for mastication, or because the 

 parts which appear to serve as jaws are not assembled an- 

 teriorly, and preceded by a labrum, as in the foregoing Crus- 

 tacea and the masticating insects, but simply formed by the 

 haunches of the locomotile organs furnished for this pur- 

 pose with small spines. The psecilopoda represent in this 

 class those which in that of the insects are distinguished under 

 the name of sncforia. Th?y are all parasite, and seem to 



VOL, XIII. Y 



