iv. 9 — iv. lo. 115 



and the Poulps, where it is very narrow, but moreover only begins 

 in the middle of the side, and does hot encircle the entire sac. 

 The use of this fin is to enable the animal to swim, and also to 

 direct its course. It acts, that is, like the rump-feathers in birds, 

 or the tail-fin in fishes. In none is it so small as in the Poulps ; 

 where it is difficult to make it out.^^ ' For in these the body is of 

 small bulk and can be steered by the feet sufficiently well without 

 other assistance.' 



All the bloodless animals, namely the Insects, the Crustacea, 

 the Testacea, and the Cephalopoda, have now been dealt with in 

 turn; and the parts of .all have been described, whether internal 

 or external. 



(Ch. \o.) We must now go back to the animals that have 

 blood, and consider such of their parts, already enumerated, 

 as were before passed over. We will take the^ viviparous animals 

 first, and, when we have done with these, will pass on to the 

 ovipara, and treat of them in like manner. 



The parts that border on the head, and on what is known as 

 the neck and throat, have been already taken into consideration. 

 As for the head- itself, such a part is found in all animals that 

 have blood ; whereas in some bloodless animals, in crabs for 

 instance, there is no head distinctly separable from the trunk. 

 As to the neck, it is present in all the vivipara, but only in some 

 of the ovipara ; for while those that have a lung also have a neck, 

 those that do not inhale the outer air have none.^ 



The head exists mainly for the sake of the brain. For every 

 animal that has blood must of necessity have a brain ; and must, 

 moreover, for reasons already given, have it placed as far as 

 possible from the heart.-^ But the head has also been chosen 

 by nature as the part in which to set some of the senses ; because 

 its blood is mixed in such suitable proportions as to ensure quiet 

 and precision to the sensations, while at the same time it can 

 supply the brain with such warmth as it requires.* There is yet 

 a third constituent superadded . to the head, namely the part 

 which ministers to the ingestion of food. This has been . placed 

 here by nature, because such a situation accords best with the 

 general configuration of the body. For the stomach could not 

 possibly be placed above the heart, seeing that this is the sovereign 

 organ ; and if placed below, as in fact it is, then the mouth could 

 not possibly be placed there also. For this would have neces- 

 686a. 



