190 CRUSTACEA CAST THEIR SHELLS. 



it to have sufficient contact with the water to allow of 

 the perfect aeration of the blood. This little creature 

 may be said to breathe through its legs and arms, 

 which may be seen in constant motion playing through 

 the fluid, and causing a constant flow of new particles 

 to the exposed surface of the blood-vessels. Can we 

 conceive a more ecstatic little being than this, whose 

 every motion is an inspiration ! at least, whose mus- 

 cular efforts bear a direct proportion to the aeration of 

 its blood ; the power that is expended in every effort 

 being renewed by the very act of making that effort. 

 In these lowest members of the class we find the legs 

 themselves performing the office of gills ; but though 

 among the higher types of structure, the legs are used 

 for the more common purposes of swimming or walking, 

 we still find the gills connected with the upper portion 

 of the legs, where they are inserted into the body. Thus 

 the active motions of walking and swimming contribute 

 to the aeration of the blood, by causing a correspondent 

 motion in the branehise. These organs are lodged in 

 two chambers, situated one at each side of the under 

 surface of the carapace or broad shelly plate, which 

 freely communicate by wide openings with the water. 

 In opening a Crab or Lobster, at table, these gills gene- 

 rally come off" attached to the bases of the smaller legs. 

 The dexterity of the Crustacea in casting their shells 

 is certainly wonderful. When one considers the hardness 

 of the shelly coat, and the extraordinary forms which it 

 assumes, especially the large claws terminating slender 

 arms, and is told that all this coat of mail is annually 

 thrown off in a single piece by the contained animal, 



