Growth-changes in Brittle Stars. 115 



radial shields are small (i. e., in the young individuals) a few spinelets 

 (3 to 6) will give them the appearance of being well covered, but as they 

 increase in size, if the number of spinelets does not increase they soon have 

 the appearance of being quite bare. In adults the shields may have very 

 few spinelets, possibly none, and no spines, but it is just as common to find 

 each shield with 10 or 12 spinelets and 3 or 4 spines. The bareness of the 

 radial shields is therefore not a reliable specific character in angulata. 



MOUTH-PARTS. 



The mouth-parts of Ophiothrix are essentially different from those of 

 Ophiactis and AmphiphoUs, since there are no oral papillae, but a group of 

 dental papillae are borne on the lower (ventral) end of the torus, below the 

 teeth. None of my specimens are young enough to show a stage of develop- 

 ment in which teeth are present but no dental papillae, though there can 

 be no possible doubt of the existence of such a stage. In the youngest 

 specimen I have examined, the torus is already expanded at its lower end 

 (pi. 3, fig. 15) and bears a group of 6 or 7 dental papillae, above which are 

 4 teeth (pi. 3, fig. 17). Resorption back of the teeth goes on as in the genera 

 already described, but in Ophiothrix the foramina thus formed are often 

 bisected by a more or less complete bar at right angles to the long axis of 

 the foramen (pi. 3, fig. 16). This indicates that the fully developed teeth 

 have two slightly separated roots and shows greater specialization than in 

 Ophiactis and AmphiphoUs. The number of teeth in angulata is 4 or 5, 

 while the number of dental papillae increases throughout growth, and a 

 large specimen may have as many as 17 (in the big East Indian Ophiothrix 

 longipeda there may be more than 30). The first dental papillae arise in a 

 pair, side by side, below the lowest tooth ; beneath them the growing torus 

 expands a little and a new papilla forms near the margin on each side ; these 

 two are thus a little separated from each other. A third pair arises below 

 them at the sides of the still expanding torus and the space thus left near 

 the center of the expanded portion of the torus is filled by the formation of a 

 seventh papilla. Subsequently, although the torus may increase much 

 in length, it does not expand greatly and the dental papillae are thus arranged 

 in two slightly curved marginal rows (one on each side) and an irregular 

 median series. In the adult, resorption has formed foramina back of the 

 upper dental papillae just as it has back of the teeth (pi. 3, fig. 16). It is 

 perfectly clear, therefore, from all these facts, that the dental papillae are 

 strictly homologous with the teeth and are not in any way to be homologized 

 with the oral papillae of other genera. As already stated, there are no oral 

 papillae in Ophiothrix^ the oral tentacles being unprotected and conspicU' 

 ously exposed on the hollowed sides of the jaw. 



ARM-BONES OR VERTEBRA. 



The vertebrae of Ophiothrix are as different from those of Ophiactis and 

 AmphiphoUs as are the jaws, yet their development is so similar as to call 



