Spines of Echinoidea. Bij Prof: F. Jeffrey Bell. 847 



in transverse sections of dried spines as expressions of its distribu- 

 tion during life.* 



With these considerations in our minds let us apply ourselves 

 to the spines of Cidarids. They are by no means so constantly 

 regular and cylindrical in this, as in other groups of Echinoids, 

 and are to be at once distinguished histologically by the presence 

 of the ostracum (the Cidaridae are the Acanthostraca of Mackintosh), 

 ■which is rarely found on the spines of other or non-Cidarid forms.f 

 Thanks, it is said, to the possession of this outer crust, the spines 

 become swollen out into irregular enlargements, or provided with 

 more or less elegant and prominent ridges, or with a broadened 

 free end. The remarkable differences in the forms of the spines 

 have been ascribed to various causes by various writers. By A, 

 Agassiz (' Revision of the Echini,' p. 653) they are said to be due to 

 " the independent growth of the outer sheath ; while in other 

 regular Echini the growth of the outer layer takes place uniformly 

 with the increase in size of the spine." This view, I take it, is a 

 modification of that held by Mr. Stewart (Q. J. M. S., xi. p. 52), 

 who is of opinion that the perisome which invests the spine dies 

 down when the outer crust of the spine has been deposited, conse- 

 quent on the acceptation of Dr. Carpenter's view as to the existence 

 of a permeating protoplasmic substance. As expressed, however, 

 it fails to convey the ideas of the author definitely to the mind, 

 and, if put into other words, such as — the outer portion of a 

 Cidarid spine grows independently of the inner — it gives an idea 

 of the minute structure of these spines, which can hardly be said to 

 agree with the facts. 



If evidence be wanting of the close relation of the inner and 

 outer regions of a spine, reference need only be made to the exact 

 figures given by Stewart or Mackintosh ; the former (op. cit.) has 

 published a sketch of a transverse section of Cidaris annulata 

 (C. tribuloides) in wJiich the dense crust is seen to be traversed by 

 delicate tubules continuous with the spaces that lie between the 

 radiating plates that form the greater part of the spine ; and the 

 latter J has given us a figure of Goniocidaris geranoides which 

 shows, as sections of its spines always do, that some of the radiating 

 spaces between the plates § extend to quite the extreme edge of the 

 periphery of the spine. 



* A student at a marine biological labtjratory might well repeat and extend 

 Dr. Carpenter's observations (op. cit.). Ho would, doubtless, derive considerable 

 aid from the copal motiiod of Koch (of. Zool. Anzeig., i. (1878) p. 36). 



t fialenki has a true crust (see fig. 5), and Echinocidaris has a crust on the tip 

 of some of its spines. 



X Trans. K. Iri.'sh Acad., xxvi. (1878) pi. ix. fig. 9. 



§ It is greatly to bo regretted that Prof. Mackintosh his followed Prof, 

 Agassiz in applying a term of such definite significance in histology as "cell " to 

 what are " really interspaces or foramina." Neither convonicucc nor the advan- 



