GEOWTH OF THE TOOTH OF ECHINUS. 
405 
mere surface growth. Again, like them, it exhibits the combination, both in its whole 
and in the anatomy of its parts, of organized form and the simplest chemical compo- 
sition — shapes elaborated under \ital forces composed of a material which physical and 
chemical actions scarcely pronounce to be other than inorganic. 
The intimate anatomy of the Echinus-tooth is in no way to be removed in essential 
character from the rest of the Echinus -skeleton. Though the plan upon Avhich its 
minute structure is formed is more symmetrical and orderly, and is more elaborate, yet 
I still concur entirely in Dr. Cabpenter’s generalization, “ that the structm’e of the 
teeth is essentially the same as that of the shell, save in the interspaces of the network 
being narrow^er.” Certain portions of the transverse section of the tooth (the skirtings) 
present coarse reticulations very closely resembling the shell-structure of the Echinus ; 
but it is by examining the humbler teeth in another Echinodeim that the absolute 
anatomical truth of this generalization is established. In the little denticles — ’■'•ggala 
angularis" of J. Mullek — the teeth of Ophiocoma, truly homologous with those of 
Echinus, the base of each exhibits an anatomical structure identical with that of the 
Echinus-shell, but gradually passing to its distal end into a tissue as entirely resembling 
the enamel of the Echinus-tooth, or the tubes betw'een the keel-fibres when cut parallel 
to their axis — the broad reticulated interspaces in the former merging into the narrow 
tubules of the latter. 
Desckiption of Plates. 
PLATE VI. 
Fig. 1. Echinus-tooth, natural size and form : a, enteric or ventral aspect ; 5, external 
or dorsal aspect. 
Fig. 2. Vertical section of portion of tooth, magnified 10 diameters. The form of the 
apex of the tooth is shown, as produced by wear and retained by the relative 
hardness of its elementary parts ; a, the clear, condensed axis of the tooth ; 
J, the body formed of plates; c, the “enamel;” and the keel. 
Fig. 3. A diagram showing the lines of direction displayed by a vertical section of — 
5, the primary plates; c, the enamel rods; and d, the keel-fibres. 
Fig. 4. Transverse section of Echinus-tooth, magnified 50 diameters. «, extremity of 
keel ; skirtings ; c, c, enamel. 
Fig. 5. The commencing growth of Echinus-tooth, originating in two systems of plates. 
This view is on the enteric or ventral surface. The figure is magnified 
100 diameters. 
Fig. 6. The same \iew as the preceding, but more advanced in development : the angles 
of the plates are modified, and the sprouting of the “ flabelliform processes ” 
has commenced. Magnified 100 diameters. 
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