482 



OF ECHINODBRMATA. 



Fig. 239. 



texture of the plates or jaws resembles that of the shell in every 

 respect, save that the network is more open ; but that of the 

 teeth is much more compact. The latter have been described 

 by Mr. Quekett as consisting of a substance not altogether un- 

 like the " dentine" of the teeth of higher animals, save that the 

 tubuli, though sometimes parallel, usually have more of a reti- 

 culated arrangement, and sometimes dilate into irregular " la- 

 cunae" or spaces excavated in the hard substance.' _ The Author 

 is not prepared to speak with confidence on this point; but he is 

 disposed to think that the structure of the teeth is essentially the 

 same as that of the shell, save in the interspaces of the network 

 being much narrower; and that the appearance of tubuli (in 

 which Mr. Quekett has not been able to make out distinct walls) 

 is due merely to the elongation of these interspaces. 



315. The calcareous plates which 

 form the less compact skeletons of 

 the Asteriada (star-fish and their 

 allies) and of the OpMurida (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 large G-oniaster 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 Ast7-o- 

 phyton (better known as the Uuryale), is represented in Fig. 239. 

 The spines with which the arms of the species of Ophiocoma 

 (brittle- star) are beset, are often remarkable for their beauty of 

 conformation ; that of 0. rosula, one of the most common kinds, 

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

 lightness and beauty, as a model for the spire of a cathedral. 



316. The calcareous skeleton is very highly developed in the 

 Orinoidea ; their stems and branches being made up of a cal- 

 careous network, closely resembling that of the shell of the 

 Echinus. This is extremely well seen, not only in the recent 

 Pentacrinus Caput Medusce, a somewhat rare animal of the West 

 Indian seas, but also in a large proportion of the fossil Crinoidea, 

 whose reniains are so abundant in many of the older geological 

 formations ; for, notwithstanding that these bodies have been 

 penetrated in the act of fossilization by a mineral infiltration, 

 which seems to have substituted itself for the original fabric (a 

 regularly crystalline cleavage being commonly found to exist iu 

 the fossil stems of Encrinites, &c., as in the fossil spines of 

 Echinidans), yet their organic structure is often most perfectly 

 preserved. In the circular stems oi Encrinites, the texture of 



' " Lectures on Histolngy," vol. ii, p. 234. 



Calcareous pl.ite and claw o{ AstropJtyton 

 (Euryalfi). 



