CRUSTACEA. 201 



drives a powerful cuiTent through the opening in 

 which it is situated. In other crustaceans, such as 

 the Mantis Crabs, the gills have the form of bunches 

 of feathers, and instead of being enclosed within the 

 thorax, float freely from the 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 terrestrial species, therefore, which breathe bv 

 means of icet gills, there exists at the bottom of the 

 respiratory cavity a sort of trough, w^hich serves as a 

 reservoir lor 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 inclosed in a sort of pouch formed of 

 appendages variously modified. Sometimes the young- 

 undergo a very remarkable metamorphosis, and not 

 only completely change their form during the earlier 

 periods of their existence, but in the progress of their 

 growth acquire additional limbs. 



All the senses of the higher animals are possessed 

 bv the Crustacea in considerable perfection. The 



K 3 



