MONOGRAPH OF THE EXISTING CKINOIDS. 633 



comatulid, it is often to be seen diving into the gut of the host, where it appar- 

 ently spends a large part of its time. He believes that it does not feed on the 

 tissues of the crinoid itself but only on the food it finds in the stomach. 



He states further that this case of commensalism is interesting because of 

 the faint but significant response to the color stimuli of the crinoid. In nearly all 

 the species of this genus where the coloration has been noted at all the chromato- 

 phores appear to be distributed over the surface so as to form a series of evenly 

 spaced dots or dashes of pigment. The concentration of these into lines and the 

 development of a purplish pigment seem to be the first effects of the commensalism. 

 The insignificant quantitative development of the pigment is probably associated 

 with the very active life of the isopod and its frequent immersion in the gut of 

 the host rather than to any incapacity of the isopod to assume the colors of the 

 environment. 



The color is white, except for branding chromatophores carrying a purple 

 pigment and arranged in two lateral stripes and a fainter median line. 



AMPHIPODA. 



In a letter written to Professor von Siebold and published by him in 1876, 

 R. von Willemoes-Suhm, writing on board the Challenger in July, 1875, during 

 the voyage from Japan to the Hawaiian Islands, mentioned that on some comatulids 

 dredged in the Arafura Sea in the preceding year a parasitic amphipod had been 

 found which had bored into the ventral disk. The specimens appear subsequently 

 to have been mislaid, for there is no mention of them in the Challenger monographs. 



On examining closely the specimens of Iridometra melpomene dredged by the 

 Albatross at a depth of 88 to 100 fathoms in the China Sea I was surprised to find 

 a number of amphipods more or less buried in the disk, which at once recalled to 

 my mind the note by von Willemoes-Suhm. I turned these specimens over to Mr. 

 Clarence E. Shoemaker, as he was so kind as to draw up a description of them for 

 inclusion in the present volume. 



While working at Murray Island, Torres Strait, Lieutenant Potts found a 

 tiny amphipod occurring very often on the darker crinoids, which proved to 

 represent a hitherto unknown genus. 



The two amphipods, one parasitic and the other symbiotic, known to be asso- 

 ciated with crinoids, are the following: 



Family AMPHILOCHID^. 



CTCLOTELSON PURPUKETTM POTTS. 

 Fig. 945, p. 621. 



This species is symbiotic. Lieutenant Potts found it common on the darker 

 comatulids at Murray Island, Torres Strait. The specimens described were taken 

 from Comanthus annulatus. 



The color dorsally is purple. 



