ON CRUSTACEA. 
269 
The onisci, aselli, and ligiae, are fond of humidity and shade, 
and most generally fix themselves under stones or in the 
breaches of rocks. 
The macrourous decapods, such as craw-fish, lobsters, 
palinurus, palaemon, or carides, as well as the entomostraca, 
are the only Crustacea which never come to land. 
We may observe, that most of the animals of this class walk 
as well as swim, their feet being adapted to both these modes 
of motion. 
The brachyurous decapods are evidently the Crustacea most 
eminently adapted for walking. In such of them as run best, 
the eight posterior feet alone are employed, and they are all 
terminated by strong and pointed claws. They walk with the 
same facility forward and backward, on one side or the other, 
or in all oblique directions possible ; they are seen to climb 
very inclined planes, and even a perpendicular surface, with 
the greatest celerity, providing that these planes be not alto- 
gether smooth. Many, such as the ocypods and gecarcini, 
are famous for the rapidity of their course, which is so great, 
that we are assured that a man would be unable to overtake 
them. 
Many brachyurous decapods walk much less than others, 
and are more decidedly aquatic. These, provided with mem- 
bers, whose articulations, flatted and ciliated on the edges, are 
transformed into true oars, can execute in the water all the 
movements which the others perform on land, and in direc- 
tions equally varied. Such are the portuni, the podapli- 
thalmi, &c. 
As for the macrouri, such as astacus and palaemon, if their 
feet serve for walking, it is only at the bottom of the water. 
Their swimming, which is usually backward, is performed by 
the motions of their strong tail, whose extremity, folded under- 
neath, is widened by laminae, which can be separated like a 
