INTERNAL STRUCTURE OF ECHINIDA. 453 



atmospheric air diffused through the waters of the ocean. The 

 fluid is introduced into the cavity by ten respiratory tubes, 

 which are placed around the mouth, and which bear a strong 

 resemblance to the tentacula of the Sea- Anemone (Fig, 61 7). In 

 the neighbourhood of the other termination of the alimentary 

 tube are situated the ovaries, which open by separate orifices 

 around the vent, just as do those of the Medusa around the single 

 aperture of its stomach. These ovaries periodically enlarge to 

 a considerable size ; and in some species they are used as food, 

 being termed the roe of the Sea-egg. In ancient Rome these 

 parts of the Echini constituted a favourite dish at the tables of 

 the great. It is not yet satisfactorily known, what degree of 

 development the germs acquire, before quitting the parent. 



1010. Many of the animals of this group are provided with 

 a very complex masticating apparatus, consisting of five hard 

 and sharp teeth, which are put in motion by a powerful set of 

 muscles ; these take their origin chiefly from five plates which 

 project around the mouth into the cavity of the shell. The 

 teeth work in bony sockets or jaws, which, when fixed together 

 in their natural positions, form a five-sided conical mass (Fig. 

 605, c, c), aptly enough compared by Aristotle to a lantern, and 

 frequently described by modern writers under the name of the 

 "lantern of Aristotle." This complex machine consists of 

 twenty-five pieces, moved by thirty-five muscles. It would 

 seem to constitute a very powerful mill, in which the food is 

 speedily reduced to fragments. Regarding the nature of their 

 aliment, however, there is still some uncertainty; but there seems 

 reason to believe, that small Crustacea and Mollusca constitute 

 their chief food ; but it is not improbable that, like others of the 

 class, they are omnivorous, and can digest almost any organic 

 matter that comes within their reach. 



1011. The two most important genera of this Order are the 

 Cidaris and the Echinus. The former is usually regarded as 

 the most highly organised of the entire class ; its spines being 

 proportionally few and powerful, and being connected with their 

 tubercles by a mode of attachment superadded to those which 

 we find elsewhere ( 1005). The Echinus, however, is the best 



