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, lobsterfe, 

 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 adajDted to both these modes 

 of motion. 



The brachyurous decapods are evidentl}' 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 podaph- 

 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 



