624 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



All of these forms are confined to the Indo-Pacific region, and nearly all to 

 the Malayan archipelago; one, S. albatrossi, occurs at Laysan Island, in the 

 Hawaiian group. 



Commensalism between an alpheid and a comatulid was first reported in 1876. 

 During the voyage of the Challenger von Willemoes-Suhm observed, in the Arafura 

 Sea, in 1874, a black and white alpheid living on a mottled black and white 

 comatulid. 



In 1882 Prof. William A. Haswell described Alpheus comatularum which had 

 been dredged by the Alert in a few fathoms of water in the Albany Passage, near 

 Cape York, Queensland, and which was invariably found clinging to the arms of a 

 comatulid, to which its color markings gave it a general resemblance. 



Mr. E. J. Miers, in the Alert report published in 1884, redescribed Alpheus 

 comatularum in great detail, and cited additional localities. It was again rede- 

 scribed under the name of Synalpheus falcatus, by Mr. C. Spence Bate in the 

 Challenger report in 1888. His specimens were dredged between Cape York and 

 the Aru Islands in 8 fathoms. 



In 1905 M. H. Coutiere recorded a second case of commensalism between an 

 alpheid and a crinoid, describing Synalpheus stimpsonii var. maldivensis, from the 

 Maldive Islands, which had been found living upon a comatulid resembling it in 

 color. In 1911 Dr. J. G. de Man described Synalpheus carinatus var. binongcensis, 

 which had been found on a crinoid, and in 1915 Lieut. F. A. Potts described Synal- 

 pheus brucei, commensal on comatulids in Torres Strait, and discusses S. coma- 

 tularum. 



SYNALPHEUS COMATDLABUM (HASWELL). 

 Fig. 1234, pi. 38. 



Lieutenant Potts writes that Synalpheus comatularum is markedly larger than 

 S. brucei and is stouter in general appearance; but the resemblance in coloration 

 and habits is so close as to suggest specific identity until the peculiar form of the 

 little chela is noticed. 



The individuals were generally seated in pairs on the disk, but when slightly 

 disturbed they immediately dug the hook of this chela into the flesh of the disk, 

 from which it was only dislodged with difficulty; or if the alpheid had left the 

 surface of the disk, the chela was serviceable for clasping the arm of its host. 

 But this instrument is only used for temporary attachment, nor is it usually 

 embedded in the host when the animal is at rest. 



Potts notes that there is, in spite of the wonderful similarity in the coloration 

 of this species and of S. brucei, one marked difference ; that is, the entrance of a 

 white or yellow pigment into the color scheme of this form. He describes a typical 

 example of a light-colored crinoid and its commensal in the following way : The 

 comatulid (Comanthus annulata) had a yellow-green disk, the arms were black 

 with white tips, sometimes with a dash of rusty-red pigment, and the pinnules 

 white, sometimes gray at the side. The underside of the arms was yellow-green 

 and the cirri were white. Both the commensals were light colored and harmonized 

 to a considerable extent with the host. The female had five prominent lines of 



