PARTS OF ANIMALS, IV. vi.-vii. 



rise in flight. This peculiarity is even more notice- 

 able in the leaping insects, such as locusts and the 

 various sorts of fleas, which first bend their hind legs 

 and then stretch them out again, and this forces them 

 to rise up from the ground. The rudder-shaped legs 

 which locusts have are at the rear only and not in 

 front ; this is because the joint must bend inwards,* 

 and no front limb satisfies this condition. All 

 these creatures have six feet, inclusive of the parts 

 used for leaping. 



VII. In Testacea the body is not divided into (/>) Testacea. 

 several parts, owing to their being of stationary 

 habits, as opposed to creatures which move about : 

 the latter are bound to have more parts to their body 

 because their activities are more numerous, and the 

 more motions of Λvhich a species is capable, the more 

 organs it requires. Now some of the Testacea are 

 altogether stationary : others move about but little ; 

 and so, to keep them safe, Nature has compassed 

 them about Avith hard shells. Some of them are (as I 

 said earlier **) one-valved, some two-valved ; and some 

 conical, either spiral like the Whelks, or spherical 

 like the Sea-urchins. The two-valved shells are 

 divided into (a) those which open — i.e. w^hich have a 

 joint on one side and can open and shut on the other ; 

 e.g. the scallops and mussels ; (6) those which are 

 joined together on both sides, e.g. the group of razor- 

 fishes. In all Testacea, just as in plants, the head is 

 dowTi below. The reason for this is that they take up 

 their food from below, as plants take it up by their 

 roots ; so they have their nether parts above and their 

 upper parts below. These creatures are enveloped 

 in a membrane, and through this they strain fresh- 



» See note on 693 b 3, p. 433. • At 679 b 16. 



349 



