CRUSTACEA. 199 



which was quite unaccountable. I thought that I could 

 perceive that the spider, before performing the above pre- 

 paratory steps, connected its legs together with most deli- 

 cate threads." 



M. Yirey has recorded some very curious observations 

 (^Bulletin cles Sciences Nat. tom. xix. p. 130), which seem 

 to prove that small spiders in an atmosphere perfectly 

 ti'anquil, and without the aid of any web, have the power 

 of darting through the air ; and believes that, by means of 

 a rapid vibration of their feet, they ivalk the air. 



" After reading M. Virey's account," says Mr. Darwin, 

 " it appears to me far from improbable that, in the case 

 above recorded, the little aeronaut actually did as was 

 suspected, unite its feet together with some fine lines, thus 

 forming artificial wings. I regret that I did not determine 

 this point with accuracy ; for it would be a curious fact 

 that a spider should thus be able to take flight by the aid 

 of temporary wings." — Voyage of the ^Beagle.' 



CHAPTER XIII. 



Crustacea. 



The Insects and Arachnidans described in the pre- 

 ceding chapters are air-breathing animals : even in 

 such species as inhabit fresli water, respiration is 

 sti'ictly aerial. Xo insects or spiders could live in 

 the sea, and, consequently, the ^Yaters of the ocean 

 would be utterly untenanted by corresponding forms 

 of Life, had not a class of beings belonging to the 

 articulated division of the animal world been so con- 

 structed as to be capable of respiring through a 

 watery medium, and thus adapted to a residence in 

 the recesses of the deep. Many species, it is true, 

 are met with abundantly in the fresh waters around 

 us ; but these form rather exceptions to the general 

 rule, and we may fairly regard the crustaceans as 

 marine representatives of the insects and spiders, 

 with which they form a parallel series. These 

 animals are divided into segments, typically twenty- 



