220 ABISTOTLE S ANAIMA, 



derms, but Prof. Forbes says that the one with large and 

 edible ova is the purple sea-egg (Echinus lividus), that the 

 Echinometra is probably E. esculentus, and that the one with 

 hard spines is Cidaris histrix; he adds that E. lividus is the 

 one chiefly used as food in the eastern Mediterranean.* 



Star-fishes are clearly referred to by Aristotle. He says 

 that they seize their prey and suck out their juices, and that 

 they destroy very many oysters.! The ravages committed 

 by star-fishes among oysters are well known. 



Aristotle says that the Aster (star-fish) is in form like a 

 drawing of a star, and makes the fanciful statement that it 

 is naturally so hot that its food is at once digested.} 



The fixed ascidians seem to have been carefully examined 

 by Aristotle. He calls them Tethya, and considers them to 

 be a special kind of his Ostrako derma. Their external 

 casing, he says, is of a nature between that of skin and that 

 of a hard shell, and can be cut like hard leather; this casing 

 is fixed to the rocks, and in it are a water inlet and a water 

 outlet. After giving this description, which is quite correct, 

 he clearly refers to the inner muscular tunic or body- wall, 

 enclosing the soft parts of the animal, but states incorrectly 

 that this tunic is a sinewy (neurode) membrane.il Again, 

 he refers to the perforated pharyngeal wall and the atrial 

 chamber through which the water, filtered from the parts 

 serving as food, passes to the water outlet. 11 



It is evident that he placed the ascidians in a far lower 

 position than they occupy in modern systems of classifi 

 cation, viz., near Amphioxus. He was not certain that they 

 deserved to be put even in his Ostrakoderma, but, concluding 

 that they had no distinct residual matters, expressed an 

 opinion that they were of the nature of plants.** 



The most interesting part of Aristotle s work in con 

 nection with his Entoma relates to his selection of the 

 animals to be included in that class. His ideas on this 

 subject were in advance of those of many naturalists from 

 his time till the end of the eighteenth century. His main 

 definition is as follows: &quot; I call those animals Entoma which 

 have incisions in their bodies, either in their ventral parts, 

 or in these and also their dorsal parts. &quot;t I 



Aristotle s definition of his Entoma is so comprehensive 



* Travels in Lycia, &amp;lt;c., 1847, vol. ii. pp. 115-6. 



t P. A. iv. c. 5, 6816. | H. A. v. c. 13, s. 10. 



Ibid. iv. c. 6, s. 1. || Ibid. iv. c. 6, s. 2. 



IT P. A. iv. c. 5, 681a. ** Ibid. ff H. A. i. c. 1, s. 7, 



