270 SUPPLEMENT 



fan. Some, such as crangon, when swimming, have the back 

 underneath, and the belly upwards. 



Man}' amphipods swim by means of the contractions of their 

 tail, aided by the movements of their feet, and some are forced, 

 in consequence of the extreme compression of their body, and 

 the very strong curvature of the tail, to remain continually 

 couched on one side. 



Although the squillse possess feet adapted for motion, they 

 appear to make no more use of them than the macrourous 

 Crustacea do of their's, and their natation appears to be effec- 

 tuated principally by the assistance of the ten branchial feet, 

 which are placed under a tail less robust and less curved than 

 that of the macrouri, but equally terminated by flabelliform 

 natatory laminae. 



In the sub-class of the entomostraca, all the animals which 

 have numerous soft feet^ and furnished with gills, as apus, 

 limnadia, and branchipes, advance only by the action of these 

 limbs, whose motions are soft and undulating. Daphnis and 

 lynceus appear to jump in the water, which has caused the 

 first to be sometimes termed aquatic fleas, because their swim- 

 ming takes place by violent movements of their ramous an- 

 tennae, which are frequently repeated, leaving but few inter- 

 vals of complete repose. In cypris, it is the feet, and espe- 

 cially the hinder feet, by which the animal advances. 



Among the amphipods, some can leap with considerable 

 vigour when they are on land, making use of their tail, folded 

 underneath, as a spring. 



The instinct, or mental faculty, of the Crustacea in general, 

 when compared with that of other classes, especially the 

 insects, appears to be but very moderately developed. The 

 crabs and their allied genera, seem to possess it in a higher 

 degree than the rest. Those animals, in fact, exhibit great cun- 

 ning, especially in avoiding their enemies : they are then ob- 



