PARTS OF ANIMALS, IV. v. 



(like limpets) it is in the tip ; in bivalves it is near 

 the hinge. In the bivalves the so-called ovum is on 

 the right-hand side, and the residual vent on the left. 

 " Ovum " is a misnomer ; actually it corresponds to 

 fat in blooded creatures when they are in good con- 

 dition ; and that is why it appears only in spring and 

 autumn, which are the seasons when they are in good 

 condition. In great cold and great heat all the 

 Testacea are hard put to it ; they cannot endure in- 

 ordinate temperatures. The behaviour of the Sea- 

 urchins is a good illustration of this : they have ova 

 in them as soon as they are born, and at the time of 

 full moon these increase in size ** ; and this is not, as 

 some think, because the creatures eat more then, but 

 because the nights are Avarmer OAving to the moon- 

 light. These creatures have need of the heat because 

 they are bloodless and therefore adversely affected 

 by cold. That is why they are in better condition 

 during the summer, and this is true of them in all 

 localities except the strait of Pyrrha,^ where they 

 flourish equally well in winter, and the reason for this 

 is that in winter they have a more plentiful supply of 

 foodstuff, due to the fish leaving the district at that 

 season. 



The Sea-urchins all have the same number of ova — 

 an odd number, five, identical with the number of 

 teeth and stomachs Λvhich they have. This is ac- 

 counted for by the " ovum " not being really an ovum 

 (as I said before) but simply a result of good nourish- 

 ment. The " ovum " is found in Oysters too, though 



see H. M. Fox, Selene, especially pp. 35 ff., and id. Proc. 

 Roy. Soc. B., 1923, 95, 523. 



" In Lesbos, leading to the lagoon, one of Aristotle's 

 favourite hunting-grounds : see Hist. An. 544 a 21 (sea- 

 urchin), 548 a 9, 603 a 21, 621 b 12. Cf. Gen. An. 763 b 2. 



329 



