CRUSTACEANS. 15 r 



false feet situated beneath the tail. In many of the lower forms, 

 the breathing organs consist of little bladders fixed to the bases 

 of the legs, while in others, the limbs themselves are so thin and 

 delicate that they seem to afford a sufficient respiratory surface. 



There are, however, some families of Crustaceans which live upon 

 dry land, and thus respire the atmospheric air, and these would 

 seem to form an exception to what has been said relative to the 

 difference of structure in the respiratory apparatus of aquatic and 

 terrestrial animals, for instead of being furnished with tracheae, 

 like the insects, they breathe air by means of gills: these, however, 

 are always disposed in such a manner as to be kept in the moist 

 state required for the exercise of their function. In these terres- 

 trial species, therefore, which breathe by means of wet gills, there 

 exists at the bottom of the respiratory cavity a sort of trough, 

 which serves as a reservoir for water sufficient to keep their 

 branchiae moist, or else the respiratory cavity is lined with a spongy 

 membrane, which seems to answer the same purpose. Others, 

 again, as the wood-lice (Oniscus), breathe a damp atmosphere, by 

 means of foliaceous appendages situated under the abdomen. 



The Crustaceans are all oviparous. The female, after having 

 laid her eggs, generally carries them about attached to the under 

 part of her body, or sometimes enclosed in a sort of pouch formed 

 of appendages variously modified. Sometimes the young under- 

 go a very remarkable metamorphosis, and not only completely 

 change their form during the earlier periods of their existence, 

 but in the progress of thejr growth acquire additional limbs. 



All the senses of the higher animals are possessed by the 

 Crustacea in considerable perfection. The organs of vision are 

 present in all at some period of their existence, and in the majority 

 of species are of a very complex structure. We find in some both 

 simple and compound eyes, similar in principle to those of insects : 

 both of these forms occur in the king-crab (LiiJiuhis), and there 

 are eyes of an intermediate character, such as that of the water- 

 flea (Dap/aria), where several clustered lenses and eye-cells are 

 covered by a single smooth and transparent cornea. But in the 

 higher forms of Crustacea, the true compound or faceted eye only 

 is met with. The facets are not always six-sided, as in insects, 

 but are sometimes square, as in the cray-fish (Astacus fluviatilis). 

 Sometimes the eyes are immoveable ; but in many species they 

 are placed at the end of jointed footstalks, of various lengths, 

 capable of being pointed in different directions ; and we often 

 find, in connection with these stalked eyes, a furrow, in which 



