CALCAREOUS TISSUE 891 



convex surface of the tooth (c, c, c) is covered with a firmer layer, 

 which has received the name of 'enamel.' This is composed of 

 shorter rods, also obliquely arranged, but having a much more 

 intimate mutual adhesion than we find among the rods of the keel. 

 The principal part of the substance of the tooth (A, b) is made up of 

 what may be called the ' primary plates.' These are triangular plates 

 of calcareous shell-substance, arranged in two series (as shown at 

 B), and constituting a sort of framework with which the other parts 

 to be presently described become connected. These plates may be 

 seen by examining the growing base of an adult tooth that has 

 been preserved with its attached soft parts in alcohol, or (which is 

 preferable) by examining the base of the tooth of a fresh specimen, 

 the minuter the better. The lengthening of a tooth below. a> it 

 is worn away above, is mainly effected by the successive addition of 

 new 'primary plates." To the outer edge of the primary plates at 

 some little distance from the base we find attached a set of lappet - 

 like appendages, which are formed of similar plates of calcareous 

 shell-substance, and are denominated by Mr. tSalter 'secondary 

 plates.' Another set of appendages termed ' flabelliform processes' 

 is added at some little distance from the growing base ; these consist 

 of elaborate reticulations of calcareous fibres, ending in fan-shaped 

 extremities. And at a point still further from the base we find the 

 different components of the tooth connected together by ' soldering 

 particles,' which are minute calcareous discs interposed between the 

 previously formed structures ; and it is by the increased develop- 

 ment of this connective substance that the intervening spaces are 

 narrowed into the semblance of tubuli like those of bone or dentine. 

 Thus a vertical section of the tooth comes to present an appearance 

 very like that of the bone of a vertebrate animal, with its lacuna?, 

 canaliculi, and lamella ; but in a transverse section the body of the 

 tooth bears a stronger resemblance to dentine ; whilst the keel and 

 enamel layer more resemble an oblique section of Pinna than any 

 other form of shell-structure. 



The calcareous plates which form the less compact skeletons of 

 the Asteroidea ('star-fish ' and their allies) and of the <>/>]iii!i-<>i<I<;i 

 ('sand-stars' and 'brittle stars') have the same texture as tlmse of 

 the shell of Echinus. And this 

 presents itself, too, in the spines or 

 prickles of their surface when 

 these (as in the great (,'oninster 

 pq'uestris or 'knotty cushion-star') 

 are large enough to be furnished 

 with a calcareous framework. An 

 exam] tie of this kind, furnished by 

 the Astrophyton, is represented in 

 fig. 679. The spines with which Pm 679 Calcareous 1)lat ,, aml elaw 

 the arms of the species ofOpMothrix of AstnmJii/tdii. 



(' brittle star ') are beset are often 



remarkable for their beauty of conformation ; those of <>. penta- 

 pJvyllwm, one of the most common kinds, might serve (as Professor 

 E. Forbes justly remarked), in point of lightness and beauty, as 



