ECHINODERMATA. 



145 



form 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 ' solder- 

 ing particles/ which are minute calcareous disks interposed between the 

 previously-formed structures; and it is by the increased development 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 lucunae, canaliculi, and lamellae; 

 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. 



FIG. 371. 



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 kee.1: B, com- 

 mencing growth of the tooth, as seen at its base, showing its two systems 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 6 its lateral portion, resembling the shell in texture; at c, c, the enamel. 



536. The calcareous plates which form the less compact skeletons 

 of the Asteriada (' star-fish' and their allies), and of the Ophiurida 

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

 shell of Echinus. And this presents itself, too, in the spines or prickles 

 of their surface, when these (as in the great Goniaster equestris) are 

 large enough to be furnished with a calcareous framework, and are not 

 mere projections of the horny integument. An example of this kind, 

 furnished by the Astropliyton (better known as the Euryale), is repre- 

 sented in Fig. 372. The spines with which the arms of the species of 

 Ophiocoma ('brittle-star') are beset, are often remarkable for their 

 beauty of conformation; those of 0. rosula, one of the most common 

 kinds, might serve (as Prof. E. Forbes justly remarked), in point of 

 lightness and beauty, as models for the spire of a cathedral. These are 

 seen to the greatest advantage when mounted in Canada balsam, and 

 viewed by the Binocular Microscope with black-ground illumination. 

 It; is interesting to remark that the minute tooth of Ophiocoma 

 10 



