io8 Papers from the Marine Biological Laboratory at Tortugas. 



jaw, for while the adambulacral plates which form the latter are still quite 

 distinct, their rudimentary beginnings may be seen (pi. 2, fig. 2). At this 

 stage there are two papillae on each side of each jaw; one is at the outer 

 (adradial) proximal corner of the first adambulacral, while the other is on 

 the inner (proximal) side of the second adambulacral, just distal to the oral 

 tentacle. Later a third papilla arises on the adradial side of the first 

 adambulacral, close behind the first papilla. The time of appearance of this 

 third papilla shows considerable diversity ; usually it forms very soon after 

 the other two, but occasionally it is delayed, and even a specimen a milli- 

 meter across may lack it (pi. 2, fig. 3). The typical generic character 

 (pi. 2, fig. 4) is commonly recognizable by the time the disk is 1.30 mm. 

 across. The existence of the stage with only the proximal and distal 

 papillae (pi. 2, figs. 2 and 3) is of extraordinary interest, for that is the con- 

 dition characteristic of the genus Amphiura and we are therefore justified 

 in saying that Amphipholis passes through an Amphiura stage. Evidently 

 light is thus thrown on the phylogeny of Amphipholis. The appearance of 

 the rudimentary oral papilla? and their relation to the first and second adam- 

 bulacral plates, with their accompanying podia, suggest the possibility that 

 the oral papillae are homologous with tentacle-scales. Investigations on 

 other genera will be necessary in order to determine the point. 



ARM-BONES OR VERTEBRAE. 



Ludwig (1881) has worked out so completely the development of the 

 vertebrae that there is really very little to be added. My observations do 

 not disagree with his in a single essential feature, although in details of 

 secondary hollows and ridges my impressions of an adult vertebra do not 

 agree with his figures. The largest vertebra I have examined is relatively 

 longer than his largest, but, as I have already stated, the Montego Bay 

 specimens of Amphipholis were all small, and their arm-segments are 

 undoubtedly relatively longer than those of larger specimens. The adult 

 vertebra may be described as somewhat elongated, with a small zygantrum, 

 a low, long protapophysis, a well-developed zygosphene, a large epana- 

 pophysis, well-developed zygapophyses, and very small adoral hypa- 

 pophyses. In all essentials it is like that of Ophiactis, but relatively much 

 longer and somewhat lower; the protapophysis is narrower, the para- 

 pophyses more conspicuous, and the epapophyses less well marked. In all 

 these particulars the vertebra of Amphipholis is more youthful than that 

 of Ophiactis. 



ARM-PLATES. 



In no particulars does the development of the arm-plates differ from 

 what has been shown for Ophiactis. The side arm-plates appear first and 

 are followed by the under arm-plate, while the upper arm-plate appears last 

 of all. In one point, however, development does not go so far in Amphi- 

 pholis as in Ophiactis. The side arm-plates are never fully separated, either 

 above or below; at least, in the largest specimen I have examined, one 



