PARTS OF ANIMALS, IV. v. 



because growth has its origin from the stomach. 

 Now if there were only one stomach, either the ova 

 would be too far a>vay from it, or the stomach would 

 entirely fill up the cavity, which would make it 

 difficult for the Sea-urchin to move about and to find 

 sufficient food to replenish itself. But, as it is, there 

 are five ova separated by five intervals, and so there 

 must be five departments of the stomach, one for 

 each interval. For the same reason there are five 

 teeth, since this enables Nature to assign one tooth 

 alike to each ovum and each department of the 

 stomach. 



I have now stated why the Sea-urchin has an odd 

 number of ova, and why it has five of them. Now 

 some Sea-urchins have quite small ones, and some 

 large : the reason for this is that the latter have a 

 hotter constitution, and the heat enables them to 

 concoct their food better. This explains \vhy the 

 uneatable ones tend to be full of residue. This 

 natural heat also induces the creatures to move about, 

 and so instead of remaining settled in one place they 

 keep on the move as they feed. An indication of 

 this is that Sea-urchins of this sort always have some- 

 thing sticking on to their spines (which they use 

 as feet),* which suggests that they are continually 

 moving about. 



The Ascidians ^ differ very little in their nature from 

 plants, but they are more akin to animals than the 

 Sponges are, which are completely plants. Nature 

 passes in a continuous gradation from lifeless things 

 to animals, and on the way there are Hving things 

 which are not actually animals, with the result that one 

 class is so close to the next that the difference seems 

 infinitesimal. Now a sponge, as I said just now, is in 



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