562 TOOTH OF ECHINUS AND OPHIOCOMA. 



be called the 'primary plates; ' these are triangular plates of cal- 

 careous 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 which has been 

 preserved with its attached soft parts in Alcohol, or (which is pre- 

 ferable) by examining the base of the Tooth of a fresh specimen, 

 the minuter the better. The lengthening of the Tooth below, as it 

 is worn-away above, is mainly affected 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. Salter ' Secondary 

 Plates.' Another set of appendages termed ' Flabelliform Pro- 

 cesses ' 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 to- 

 gether by 'Soldering Particles,' which are minute Calcareous disks 

 interposed between the previously-formed structures ; and it is the 

 increased development of this connective substance, which narrows 

 the intervening spaces into the semblance of tubuli resembling 

 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 Ver- 

 tebrate animal, with its lacunae, canaliculi, and lamella? ; but in a 

 transverse section the body of the tooth bears a stronger resem- 

 blance to Dentine ; whilst the keel and enamel-layer more resemble 

 an oblique section of Pinna than any other form of Shell-structure. 

 — It is interesting to remark that the gradational transition be- 

 tween the ordinary reticular structure of the Shell, and the dentine 

 and enamel-like substance of the Tooth, which can only be traced 

 in the adult Tooth of the Echinus by examining it near its base, is 

 most distinctly presented by the tooth of Ophiocoma, which is so 

 minute that it may be mounted in Balsam as a transparent object 

 with scarcely any grinding-down, and which then shows that the 

 basal portion of the Tooth is formed upon the open reticular plan 

 characteristic of the 'Test,' but that this is so modified in the 

 older portion by subsequent addition, that the upper part of the 

 Tooth has the bone-like character of that of the tooth of Echinus. 

 434. The Calcareous Plates which form the less compact Skele- 

 tons 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 Astro- 

 phyton (better known as the Euryale), is represented in Fig. 292. 



