94 The Echinus or Sea Urchin. 



in a circle and ultimately prevent the animal from using its mouth. 

 Instances where this has happened are not unfrequently met with. 



Now each of the five teeth of the echinus grows in a precisely 

 similar way. It is true that from the outside merely the points are 

 seen, but these teeth are of considerable length, and though their 

 structure is not so complicated as those in the rodents (rats, &c), 

 yet it is continually growing, new matter being continually added 

 by a " sac w at the other extremity. It will be impossible to convey 

 any idea of the dental apparatus without a figure. 



Each tooth lies in an alveolus; this alveolus may be best described 

 as a hollow triangular pyramid, placed point downwards, and of 

 such a size that the five alveoli, when placed side by side, just form 

 a complete cone. The edges of the alveoli which would meet inside 

 are sliced off, thus leaving a space for the passage of the gullet. 

 Between each alveolus at the upper (broader) end is a small piece 

 called a rotula, which serves to join two adjacent alveoli, and again 

 from each rotula springs a small pointed piece called a radius, which 

 meeting the other radii in the centre, serves to keep the alveoli still 

 more in position. The whole affair looks something like a lantern, 

 and, from the name of its discoverer, Aristotle, has been called the 

 Lantern of Aristotle. Round the interior of the oral aperture, and 

 fastened to the corona, are five arches called auricula, and to these 

 auriculae the Lantern of Aristotle is attached by a very complicated 

 set of muscles, which also serve to move the alveoli, &c, so as to 

 enable the teeth to work against one another. 



From such a wonderfully complicated apparatus, we should 

 naturally suspect that the animal made very good use of its teeth, 

 and it would therefore be no very difficult matter to find out what 

 it really did eat. Hitherto this has not been the case. The 

 " stomach " has only been noticed to be filled with sand and broken 

 "pieces of shell and coral, and hence it is suspected that it lives 

 upon mollusca and Crustacea. 



As regards the development of the echinus, from the extraordinary 

 forms presented by the young of other lowly- organised animals, we 

 might expect great differences in appearance and structure as the 

 animal grows from its very young to adult state, and this is the 



