74 CLASS VI. CRUSTACEA. 



situated at the lower extremity of their bodies, which are 

 perforated with an immense number of little holes. By means 

 of their webs, many species of spiders, particularly when 

 young, are able to transport themselves to a considerable dis- 

 tance through the air. In order to effect this, they ascend 

 some eminence, and throw out a number of webs ; these are 

 raised up and carried along by the wind, and the animal, be- 

 ing buoyed up by them, is conveyed sometimes to a great 

 height. In order to alight, they have only to disengage them- 

 selves from a part of their web, and suffer themselves to de- 

 scend gradually to the ground. It is probable that they have 

 recourse to this expedient, in part at least, for the purpose of 

 catching insects for food. In autumn, the air is often full of 

 the, cobwebs which have been made use of for this singular 

 mode of conveyance; and those who have ascended emi- 

 nences for the purpose of observing this phenomenon, have 

 frequently seen spiders floating by in the air, supported in the 

 manner just now described. 



Many branches of this family are exceedingly cruel and 

 ferocious, not sparing even their own species. The bite of 

 many of them is poisonous, particularly that of the tarantula 

 and the scorpion. They undergo no metamorphosis, but shed 

 their skins several times. A few receive an additional pair 

 of legs at some time after birth. 



SECTION VII. 

 Class VI. Crustacea. 



THE Crustaceous animals have been sometimes included in 

 the class of insects, to which they have indeed many strong 

 points of resemblance. They deserve, however, a separate 

 consideration, both on account of their size and importance, 

 and of some anatomical differences of structure, which will be 

 pointed out. Among the most familiar examples of this class 

 are the lobster, crab, crawfish, and what is usually called the 

 horse-shoe. '! 



They have articulated limbs, antennae, and jaws, similarly 

 formed to those of insects. But they breathe by means of 

 branchiie or gills, and have a regular double circulation ; in 

 which particulars they differ from insects. The blood which 

 has passed through the gills, is collected into one large vessel, 



