GILLS OF CRUSTACEA. 189 



Crustacea, being either water animals, or constantly 

 frequenting very damp places, respire through the me- 

 dium of gills. This is one important feature in their 

 economy by which they differ from insects. Another is, 

 that they continue to increase in bulk after they have 

 attained the mature form of their kind. In the insect, 

 increase in bulk, and the changes of skin which it re- 

 quires, are confined to the metamorphic stages through 

 which the animal passes, and cease when the limbs 

 acquire their permanent form. In the Crustacean, 

 though the earlier stages undergo metamorphoses, some 

 of them quite as singular as those that we find among 

 insects, the animal continues to increase in size long 

 after its limbs have been completely formed ; and the 

 provision by which this is effected is not the least 

 curious point in their history. Differences of this im- 

 portant nature, and others of a similar kind, added to 

 the immense extent of both classes, have induced mo- 

 dern naturalists to separate the class Crustacea from 

 the other articulate animals with which Linnaeus com- 

 bined them. 



The different aspects which the gills assume in the 

 various groups of Crustacean animals, while they afford, 

 as in other classes, excellent classifying characters, ex- 

 hibit to us some beautiful adaptations, which are quite 

 as interesting to the unlettered observer as to the sys- 

 tematic naturalist. In some of the more minute indi- 

 viduals of the race, as in the Water-fleas, which may be 

 found by myriads in any stagnant pool, the respiratory 

 organs are seated in the legs themselves, whose covering 

 is so delicate that it admits the vessels that ramify over 



