Growth-changes in Brittle Stars. 107 



MOUTH-PARTS. 



My observations on the mouth-parts confirm fully the work of Fewkes 

 and Ludwig. I am, however, able to add certain details in regard to the 

 teeth and oral papillae which were either overlooked by those writers or 

 considered of too little importance to note. As mentioned under Ophiactis, 

 the torus forms at the tip of the jaw as a small plate about as wide as high, 

 but rapidly becomes vertically elongated. Before it is twice as high as long, 

 the first tooth is formed from a triradiate spicule lying close to the torus 

 on its inner side, near its upper end. This spicule rapidly forms a triangular 

 pyramid, the base of which rests very closely against the torus (pi. 2, fig. 5). 

 Fewkes says that "the teeth are not separate centres of calcification," but 

 appear "to grow out directly from the adoral region of the torus." My 

 first impressions were that this statement was correct, so closely are the 

 teeth associated with the torus, but careful maceration with sodium hypo- 

 chlorite made it possible to determine the truth. So closely does the young 

 tooth press against the torus, that resorption (as stated under Ophiactis) 

 takes place in the latter and a socket is formed in which the tooth sets very 

 snugly, but probably not immovably. The first-formed tooth attains some 

 size (pi. 2, fig. 6) before the second is formed. As the torus elongates 

 vertically the second tooth arises below the first and subsequently a third 

 forms beneath that, and finally a fourth below that; the maximum number 

 in Amphipholis from Montego Bay seems to be 4; probably larger specimens 

 will be found to have 5. The growth of the torus seems to be chiefly at the 

 lower end, and that of the teeth is mainly distal. In the adult torus the 

 sockets, in which the teeth are set, completely perforate its skeletal tissue, 

 with the occasional or perhaps usual exception of the lowest. The torus 

 is widest and thickest at the top. 



Fewkes has laid considerable stress on the formation of a spinous pro- 

 jection at the distal end of the second adambulacral plate, during the 

 formation of the jaw, which he considers homologous with an arm-spine. 

 There is no doubt of the existence of this process. Ludwig and others 

 refer to it and I have had no difficulty in finding it (pi. 2, fig. i). But in 

 following its development we find it never becomes a spine or any other 

 special organ; it simply forms the distal, concealed end of the adoral plate, 

 to which the second adambulacral plate gives rise. We have already seen 

 under Ophiactis that the arm-spines arise from separate centers of calci- 

 fication, but neither Fewkes nor Ludwig noted this fact. Had it been 

 observed by Fewkes, he could hardly have homologized the distal process 

 of the adoral plate with an arm-spine; he seems to have been misled by a 

 superficial resemblance and to have overlooked the important fact that 

 spines do not arise by budding or breaking off from previously formed 

 plates. 



The origin and development of the oral papillae in Amphipholis is of 

 especial importance, since their appearance in the adult is the character upon 

 which the genus is based. They arise very early in the development of the 



