890 



ECHINODERMA 



Another example of the same structure is found in the peculiar 

 framework of plates which surrounds the interior of the oral orifice 

 of the shell, and which includes the five teeth that may often be seen 

 projecting externally through that orifice, the whole forming what 

 is known as the ' lantern of Aristotle.' The texture of the plates 

 or jaws resembles that of the shell in every respect, save that the 

 network is more open ; but that of the teeth differs from it so widely 

 as to have been likened to that of the bone and dentine of vertebrate 

 animals. The careful investigations of Mr. James Salter, 1 however, 

 have fully demonstrated that the appearances which have suggested 

 this comparison are to be otherwise explained, the plan of structure 

 of the tooth being essentially the same as that of the shell, although 

 greatly modified in its working out. The complete tooth has some- 



FIG. 678. Structure of the tooth of Echinus : A, vertical section, showing 

 the form of the apex of the tooth as produced by wear, and retained by 

 the relative hardness of its elementary parts ; a, the clear condensed axis ; 

 b, the body formed of plates ; c, the so-called enamel ; d, the keel. B, 

 commencing growth of the tooth, as seen at its base, showing its two sys- 

 tems of plates ; the dark appearance in the central portion of the upper 

 part is produced by the incipient reticulations of the flabelliform processes. 

 C, transverse section of the tooth, showing at a the ridge of the keel ; at b 

 its lateral portion, resembling the shell in texture ; at c c, the enamel. 



what the form of that of the front tooth of a rodent, save that its 

 concave side is strengthened by a projecting ' keel,' so that a trans- 

 verse section of the tooth presents the form of a _|_. This keel is 

 composed of cylindrical rods of carbonate of lime, having club-shaped 

 extremities lying obliquely to the axis of the tooth (fig. 678, A, d) ; 

 these rods do not adhere very firmly together, so that it is difficult 

 to keep them in their places in making sections of the part. The 



essay in Ann. and Mag. Nat. Hist. (5),vi. p. 101 ; andM. Foettinger's paper in vol. ii. 

 p. 455 of the Archives de Biologic. 



1 See his memoir, ' On the Structure and Growth of the Tooth of Echinus,' in 

 Phil. Trans, for 1861, p. 387. See also Ofiesbrecht, ' Der feiiiere Bau der Seeigel- 

 zk'hne,' Morph. Jahrbuch, vi. p. 79. 



