96 Papers from the Marine Biological Laboratory at Tortugas. 



true apparently of ophiurans, the only soft parts which show any evident 

 modification without corresponding skeletal changes being the podia (tube- 

 feet), and even they show relatively little diversity. 



The investigations have been strictly confined to growth-changes, as 

 indicative of relationships within the class. I have intentionally avoided, 

 on the one hand, any discussion of histological changes, partly because this 

 was aside from the ends in view and partly because such changes have been 

 so ably and fully described for Ophiactis by Simroth (1876). On the other 

 hand, I have not entered into any discussion of the homologies of the 

 skeleton in ophiurans as compared with starfishes, nor of the homologies 

 of the jaws, as compared with the arms, since Ludwig's (1878, 1879, 1880, 

 1 881) work has rendered any such contribution on my part quite super- 

 fluous. Suffice it to say that in all of my work I have had the homologies 

 suggested by our Nestor of echinoderm morphology constantly in mind, 

 and I have seen nothing whatever to cast any doubt upon his interpretations 

 of ophiuran structure, with the possible exception of the homology of the 

 peristomal plates. On this question I do not care at present to express an 

 opinion. 



RESULTS. 



The facts which I have been able to ascertain will undoubtedly be more 

 plainly set forth if each of the three species is treated by itself. Ophiactis 

 and Amphipholis are unfortunately rather closely related to each other, 

 the morphological differences between them being of little importance. 

 Ophiothrix, on the contrary, is quite different and shows some interesting 

 peculiarities. As Ophiactis seems to be undoubtedly less specialized than 

 Amphipholis, it may well be the first genus discussed. 



Ophiactis savignyi (Miiller and Troschel). 



The large number of localities from which this species has been recorded, 

 led me to give special attention to the identification of the specimens 

 from Montego Bay. I have compared them with specimens, most of them 

 identified by Lyman, from Zanzibar, Mauritius, Singapore, the Philippine 

 Islands, the Pelew Islands, Fiji, Hawaii, Lower California, Panama, Florida, 

 the Bahamas, the Bermudas, and Brazil. Absolutely indistinguishable 

 in every essential particular, these numerous specimens prove that this 

 species is truly tropicopolitan. It is one of the small species of the genus, 

 the disk-diameter of adults being usually about 5 mm. and very rarely 

 exceeding 7 mm. There are usually 6 arms, but about one specimen in 

 ten is perfectly pentamerous. In some lots the proportion of pentamerous 

 specimens runs considerably higher than this, while in others the hexam- 

 erous symmetry is almost unvaried. I have failed to find any correlation 

 between the pentamerous symmetry and any other character of the indi- 

 vidual, nor is it any more frequent in adult specimens than in the young. 

 In addition to its hexamerous symmetry and small size, savignyi is char- 



