PARTS OF ANIMALS, IV. v. 



on one side of the body only ; it is the same as that of 

 tlie Sea-urchin. Now the Sea-urchin is spherical, 

 and is not just one flat disk hke the Oysters ; thus, 

 being spherical, it is not different shapes in different 

 directions, but equiform in all directions ; hence of 

 necessity its " ovum " is correspondingly arranged, 

 since this creature's perimeter is not, as in the others, 

 non-equiform " : they all have their head in the 

 centre, whereas the Sea-urchin's is at the top. 

 Yet even so the " ovum " cannot be continuous, 

 since no other of the Testacea has it thus ; it 

 is always on one side of the disk only. Hence, 

 since this is a common property of all species 

 of Testacea, and the Sea-urchin is peculiar in having 

 a spherical shape, the result follows of necessity that 

 the Sea-urchins cannot have an even number of ova. 

 If they were even, they would have to be arranged in 

 diametrically opposite positions, because both sides 

 would have to be alike, and then there would be ova 

 on both sides of the circumference ; but this arrange- 

 ment is not found in any of the other Ostreae ; both 

 Oysters and Scallops have ova on one side only of 

 their circumference. Therefore there must be three, 

 or five, or some other odd number of ova in the Sea- 

 urchin. If there were three, they would be too far 

 apart ; if more than five, they would be quite con- 

 tinuous ; the former would not subserve a good 

 purpose, the latter is impossible. Therefore the 

 Sea-urchin must of necessity have five ova. 



For the same cause the creature's stomach is cloven 

 into five and it has five teeth. Each of the ova, being, 

 as it were, a body belonging to the creature, must 

 conform to the general character of the stomach, 



• That is, it is circular in all planes, not in one only. 



331 



