iv. 5. 103 



as to keep the necessary symmetry ; one ovum of each pair being 

 placed at one end, the other ovum at the other end, of a transverse 

 diameter. This again would violate the universal provision in 

 Testacea. For both in the Oysters and in the Scallops we find the 

 ovum only on one side of the circumference. The number then 

 of the ova must be an uneven one, three for instance or five. But 

 if there were only three, they would be much too far apart ; while 

 if there were more than five, they would come to form a continuous 

 mass. The former arrangement would be disadvantageous to the 

 animal, the latter an impossibility. There can, therefore, be 

 neither more nor less than five. For the same reason the stomach 

 is divided into five parts, and there is a corresponding number 

 of teeth. For seeing that the ova represent each of them a kind 

 of body for the animal, from which the materials for growth are 

 derived, there must be a conformity between them and the general 

 configuration of the vital organs. Now, if there were only one 

 stomach, this conformity would be wanting. For either the ova 

 would be too far off from the stomach, or the stomach would be 

 so big as to fill up the whole cavity, and the Echinus would have 

 great difficulty in moving about, and finding due nourishment for 

 its repletion. As then there are five intervals between the five 

 ova, so are there of necessity five divisions of the stomach, one 

 for each interval. So also and on like grounds there are five 

 teeth. For by this arrangement nature is enabled to allot to 

 each stomachal compartment and ovum its separate and similar 

 tooth. These then are the reasons why the number of ova in 

 the Echinus is an odd one, and why that odd number is five. 

 In some Echini the ova are excessively small, in others of con- 

 siderable size, the explanation being that the latter are of a 

 warmer constitution, and so are able to concoct their food more 

 thoroughly ; while in the former concoction is less perfect, so 

 that the stomach is found full of residual matter, while the ova 

 are small and uneatable.^ Those of a warmer constitution are, 

 moreover, in virtue of their warmth more given to motion, so that 

 they make expeditions in search of food, instead of remaining 

 stationary like the rest. In proof of this, it will be found that 

 they always have something or other sticking to their spines, 

 which they use as feet in their frequent ramblings.^^ 



The Ascidians differ but slightly from plants and yet have 

 more of an animal nature than the Sponges, which are in 

 681a. ■, 



