viii ECHINODEBMATA— ARISTOTLE'S LANTERN 401 



ends. The lower end, which projects beyond the tip of the pyramid, is short and 

 pointed, and forms the externally visible part of the tooth lying in the mouth. The 

 upper end, which is directed aborally, is called the root of the tooth, and projects 

 considerably beyond the foramen basale, it is usually coiled inwards (towards the 

 axis of the masticatory framework). The growth of the tooth no doubt takes place 

 principally at this root end. On its inner side, the tooth usually has a longitudinal 

 ridge, the carina, and on its outer side is firmly attached to the outer wall of the 



Fio. 347.— Masticatory apparatus of an Echinus, original. A, In profile. B, From tlie apically 

 directed basal .side. C, External view of a single pyramid. D, Side view of the same. E, Internal 

 view of the same. F, Tooth. 1, Arcus ; 2, intermediate plate ; 3, freely projecting portion 

 of the teeth ; 4, median portion of a tooth ; 5, upper portion of the same ; 6, the limbs of a forked 

 radius (7) ; 8, single pyramid in situ. 



pyramid which it traverses, in .such a way that it cannot move by itself, but only 

 with its pyramid. 



The fine structure of the teeth differs essentially from that of the other skeletal 

 pieces of the body (cf. on this subject the special treatises mentioned in the Biblio- 

 graphy). 



The iutermediate plates are five more or less flat, oblong, skeletal masses lying 

 on the base of the masticatory apparatus, like the spokes of a wheel round its central 

 axis. Each of these intermediate plates rests on the bases of the tAvo contiguous 

 lateral walls of two pyramids [or pairs of jaws], and therefore between two foramina 



Finally, lying apically upon these falces, are the five forked radii, which are also 

 arranged like the spokes of a wheel. Each radius consists of a slender central 

 stalk, and of two peripheral diverging prongs, and each is bent downwards in such a 

 VOL. II 2d 



