CKUSTACEA. 385 



Hydrachne, or Water-mites, for example, at their birth present them- 

 selves under the form represented in fig. 194 b , 3, being at that time pro- 

 vided with only six locomotive limbs and a very remarkable proboscis*. 

 These larvae at first swim at large in the water, but at length contrive 

 to fix themselves to the body of some aquatic insect, in which position 

 they pass into the state of nymphs (fig. 194 b , 4) ; the hinder part of 

 their bodies becomes remarkably elongated, and at length assumes 

 the form of a pear, in which all resemblance to its former state is lost. 

 Nevertheless, during this remarkable increase in size, the proboscis 

 and the legs undergo but little alteration ; for as soon as the body 

 begins to elongate, the legs and the palpi are withdrawn from their 

 original integument, and, retiring with the body into the pear-shaped 

 sac formed by the distended skin, nothing is left behind but the exu- 

 viated sheaths of the old legs, which are then easily broken off by the 

 slightest violence. A nymph, formed in the interior of its own skin, 

 has replaced the larva ; but it is a nymph which continues to nourish 

 itself, and to increase in size, until it assumes the appearance shown 

 at fig. 194 b , 5, in which the rudiments of a new set of legs are 

 clearly perceptible through the transparent envelope : the eyes of the 

 contained animal are distinctly traceable, and may even be seen to 

 abandon their former corneae, and to recede in the same proportion as 

 the limbs from the old case, which at last, rending transversely into two 

 portions, allows the new animal to escape, which immediately begins to 

 swim vivaciously about, under a form closely resembling that of the 

 parent. It has, however, still another moult to go through before it 

 can be pronounced adult ; for, after having lived some weeks in this 

 third condition, and visibly increased in size, these immature individuals 

 fix themselves to some water-plant, to which they hold firmly by means 

 of their beak and claws, and, becoming motionless, again exuviate 

 (fig. 194 b , 8), and are ready to reproduce their kind. 



CHAPTER XIV. 

 CKTJSTACEA. 



(981.) INSECTS and Arachnidans are air-breathing animals; and even 

 in such species of these two extensive classes as inhabit fresh water, 

 respiration is strictly aerial. No insects or spiders are marine ; and con- 

 sequently the waters of the ocean would be utterly untenanted by cor- 

 responding forms of Articulata, were there not a class of beings belong- 



* Duges, Ann. des Sci. Nat 2 ser. torn. i. p. 165. 



2c 



