102 iv. 5- 



and autumn. For no Testacea can abide extremes of temperature, 

 and they are therefore in evil plight in winter and in summer. 

 This is clearly shown by what occurs in the case of the Echini. 

 For though the ova are to be found in these animals even directly 

 they are born, yet they acquire a much greater size than usual at 

 the time of full moon ; • not, as some think, because the Echini eat 

 more at that season, but because the nights are then warmer, owing 

 to the moonlight.*^ For these creatures are bloodless, and so are 

 unable to stand cold and require warmth. Therefore it is that 

 they are found in better condition in summer than at any other 

 season ; and this all over the world excepting in the Pyrrhean 

 tidal strait. There the Echini flourish as well in winter as in 

 summer. But the reason for this is that they have a greater 

 abundance of food in the winter, because the fish desert the strait 

 at that season.*^ 



The number of the ova is the same in all Echini, and is an odd 

 one. There are in fact five ova, just as there are also five teeth 

 and five stomachs ; and the explanation of this is to be found in 

 the fact that the so-called ova are not really ova, but merely, as 

 was said before, the result of the animal's well-fed condition. 

 Oysters also have an ovum, corresponding in character to that of 

 the Echini, but existing only on one side of their body.^^ .Now 

 inasmuch as the Echinus is of a spherical form, and not merely 

 a single disk like the oyster, and in virtue of its spherical shape 

 is the same from whatever side it be examined, it follows as a 

 necessary consequence that its ovum must be of a corresponding 

 symmetry. For the spherical shape has not the asymmetry of 

 the disk-shaped body of the Oysters. For though the head in 

 all of these is central, yet their ovum is on one side, namely the 

 upper."*® Now the necessary symmetry would be observed, were 

 the ovum to form a continuous ring. But this may not be. For 

 it would be in opposition to what prevails in the whole tribe of 

 Testacea ;^^ for in all the ovum is discontinuous, and in all, 

 excepting .the Echini, asymmetrical, being placed only on one 

 side of the body. Owing, then, to this necessary discontinuity 

 of the ovum, which belongs to the Echinus as a member of the 

 class, and owing to the spherical shape of the body, which is its 

 individual peculiarity, this animal cannot possibly have an even 

 number of ova. For were they in even number, they would 

 have to be arranged exactly opposite to each other, in pairs,. so 

 680b. 



