478 STRUCTURE AND DEVELOPMENT OF TEETH, W. WALDEYER. 



again, like the papillae fungiformes, are beset with small pro- 

 jections (Kolliker, 58). No glands appear to be present in 

 them. Here and there small round heaps of pavement epi- 

 thelium, frequently presenting the appearance of concentric 

 lamellae of horn, are met with, either imbedded in the sub- 

 stance of the gum, or occupying fossae on its surface (Serres, 8 ; 

 Kolliker, 58). The periosteum of the alveoli, which fulfils 

 the office of periosteum, not only to the internal surface of 

 the alveolus, but also to the cement, termed the Periodontium, 

 is characterised by its -softness. It contains but few elastic 

 fibres, though I, with Kolliker (58), have found its nervous 

 supply abundant. 



Dentinal structures occur in large numbers, and present a great 

 variety of form, amongst the Invertebrata. The teeth of the mastica- 

 tory apparatus of the Echinus most closely resemble those of the 

 Vertebrata. H. Meyer" states that they are composed of enamel 

 fibres ; this, however, is not quite accurate. The teeth of the EchinidaB 

 are long, slender, slightly curved plates, which present .a well-marked 

 longitudinal ridge on their inner surface. The greater part of each 

 tooth is formed by a radial lamina attached vertically to the surface 

 of this ridge or keel. The radial lamina is moderately soft, and can 

 be easily broken up into thin leaflets, which are again composed of 

 elongated prisms somewhat curved at their extremities. The peri- 

 pheric plate is considerably harder, and its prisms are much smaller 

 and softer than tbose of the keel. Between these prisms, which in 

 part run parallel to one another, and partly decussate in each plate, 

 lie thin lustrous calcareous plates which often exhibit .an extremely 

 delicate plexus of fine anastomosing canaliculi. When treated with 

 hydrochloric acid, the prisms dissolve with the disengagement of a 

 large quantity of gas, and leave no organic residue. They appear, 

 therefore, to be -entirely composed of carbonate of lime. In their 

 degree of hardness, in their size and chemical characters, tbey con- 

 sequently differ remarkably from true enamel, ,and they do not 

 possess the regular four or six-sided form, characteristic of the 

 fibres of the latter substance. In Mollusks, Worms, and Arthropods 

 the oral or gastric teeth are composed of chitine, which is sometimes 

 impregnated with lime .or silica. It may be said generally that the 

 teeth amongst the Invertebrata are to be regarded as pure mineral 



* Miiller's Archiv, 1849, p. 191, et seq. 



