Miscellanea Zoologica. 375 
breathe by gills or branchiae, with others which have no special or- 
gans for the performance of that important function, being reduced 
to respire by the skin ; animals which have a heart and a very 
complicated vascular system, with others which have no heart and 
no very distinct vessels ; animals which have a powerfully armed 
mouth and^reatly developed chylopoetick viscera, with others which 
are sucklings, and whose intestines are fitted for the'digestion of no 
solid food. But these difficulties disappear when we discover how 
much these organs, so influential in the higher animals, are modified 
before they entirely disappear in these less perfect beings ; how 
they become by little and little rudimentary previous to their obli- 
teration, a loss which is then little felt, and is not accompanied with 
any essential change in the type of their organization. Branchiae, 
for example, become rudimentary and disappear, to be replaced and 
have their functions performed, by the common integuments in some 
crustaceans, almost alike, in other respects, to others which are fur- 
nished with these organs in a state of high developement, and that 
too without any notable modification in the other great systems. The 
blood-vessels cease to have distinct parietes, and exist no longer ex- 
cepting as simple lacunae in some Crustacea, which it is impossible to 
separate from other animals of the same class having a very complete 
vascular system ; and the heart becomes rudimentary, and perhaps 
even completely disappears, although nothing has occurred in the 
body generally to reveal its absence. These facts have induced M. 
Edwards rightly to place in the class Crustacea not only the arti- 
culated animals with jointed feet, having a complete circulation and 
branchiae the characters which are usually assigned to it, but al- 
so all those which, formed on the same general plan, are more or 
less imperfect, and in one sense degraded. The group thus form- 
ed will be more difficult to define, but better this than that it should 
be circumscribed by arbitrary limits. * 
Class CRUSTACEA Sub-class C. HAUSTBLLATA. 
Order, ARANEIFORMES, M. Edwards, 
(Podosomata, Leach. Pycnogonides, Latreille.) 
Character. Animals crustaceous, araneiform : head rostrate, tu- 
bular, the mouth terminal, simple : thorax linear, of 4 sub-equal 
segments, the anterior with 4 simple eyes placed on a dorsal tuber- 
cle : legs 8, exclusive of the auxiliary organs to the head, very long 
proportionably, ambulatory, raptorious, 8-jointed: abdomen rudi- 
* Hist. Nat. des Crust, i. 227-8, 
