

316 CRUSTACEA. 



tic. Those which are terrestrial, like all other Crustacea which 

 live out of water, still require a certain degree of atmospheric hu- 

 midity to enable them to breathe, and to preserve their branchiae in 

 a proper state for the exercise of that function. 



This order according to the system of Linnaeus embraces the 

 genus 



ONISCUS, Lin., 



Which is now divided into six sections. Those of the first are parasitical. 

 The subgenera are numerous. Some are aquatic and others terrestrial. 

 Among the latter we have the 



Oiascus, Lin. 



The true Onisci have eight joints in their lateral antennae, but their base is 

 covered, and the two external appendages of the extremity of the tail are 

 much larger than the others. These animals are vulgarly called Clous-d- 

 porte, and by syncope Cloporte, Porcelets, de Saint-Antoine, Wood lice, &c. 

 They inhabit retired and obscure places, cellars, fissures in walls, old build- 

 ings, under stones, &c., &c. The^feed on decaying vegetable and animal 

 matters, and seldom issue from their retreat, except in rainy weather. 

 They move but slowly, unless they are alarmed. The ova are inclosed in a 

 pectoral pouch. The young, at birth, have one thoracic segment less than 

 the adult, and consequently have but twelve feet. 



SECOND GENERAL DIVISION. 



ENTOMOSTRACA. 



Under this denomination, which is taken from the Greek, and sig- 

 nifies Insects with shells, Othon Frederick Muller comprises the 

 genus Monoculus of Linnaeus, to which we must add some of his 

 Lerneae. 



These animals, mostly microscopic, are all aquatic, and mostly in- 

 habit fresh water. Their feet, the number of which varies, and that 

 sometimes extends to beyond a hundred, are usually fitted for nata- 

 tion only, being sometimes ramified or divided, and sometimes fur- 

 nished with pinnuloe or formed of lamellae. Their brain is formed 

 of one or two globules. The heart has always the figure of a long 

 v-essel. The branchiae, composed of hairs or setae, singly or united, 



