462 INTERNAL STRUCTURE OF ECHINIDA. 



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. 719). 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. 



1100. 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. 

 707, 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 yery powerful mill, in which the food is 

 speedily reduced to fragments. Regarding the nature of their 

 aliment, however, there is still some uncertainty ; 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. 



1101. Perhaps the most singular fact in the economy of the 

 Echinida, although something of the same kind is presented by 

 several other species of the class, is the mode of their develop- 

 ment from the egg. The egg gives birth to a minute embryo 

 clothed with cilia, by the agency of which it swims freely about 

 in the water. This, which is at first globular, gradually ac- 

 quires a pyramidal form, and its base presents an opening or 



