CLASS CRUSTACEA. 
321 
furnished with pinnulse, 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 branchiopoda , which we have applied to these animals, 
of which at first we had formed but a single order. Almost all 
of them have a testa, 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, 
ihe second jaws, those of the phyllopoda, at most, excepted, 
even resemble these latter organs. Jurine has sometimes de- 
signated them under 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 with 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 paecilopoda represent in this 
class those which in that of the insects are distinguished under 
the name of suctona. They are all parasite, and seem to 
VOL. XIII. Y 
