ON TWO SPECIES OF HYDRACTINIA LIVING IN 
SYMBIOSIS WITH A HERMIT CRAB 
SEITARO GOTO^ 
Professor of Zoology, Imperial University, Tokyo 
TWENTY-THREE FIGURES 
Two species of H3^dractinia have been known among us for 
many years, which, though very different in external appearance, 
have the same habit of Uving always in symbiosis with a hermit 
crab, Eupagurus constans Stimpson, and of forming shells'' 
of their own entirely composed of a chitinous framework, so that 
in most specimens there is apparently no basis of gastropod shell, 
as is the case in most other known species of Hydractinia. The 
skeleton of one of the species is totally devoid of spines and its 
substance is ver}^ thin and papery, while that of the other is 
richly armed with large spines, which are conical when small 
but irregular in shape and branching when large. The latter 
species has recently been described by Stechow from the Doflein 
collection and identified by him with the Hydractinia sodalis 
of Stimpson^ who in his description of Eupagurus constans, men- 
1 After writing the above I received a valuable letter from Dr. Stechow, to 
whom I had written concerning this hj'droid, giving some further information 
in regard to its literature. According to him the hydroid and the hermit-crab 
are treated on pp. 218-220 of the paper entitled, "Report on the Crustacea 
collected by the North Pacific Exploring Expedition, 1853-1856," by W. Stimp- 
son, recently published in vol. 49 of the Smithsonian Miscellaneous Collection, 
1907, the discovery of the paper being due to Miss Rathbun. According to 
Dr. Stechow the description in the text and the good figure reproduced in 
pi. 24 leave no doubt that Stimpson had before him the Hydractinia with large 
spines. For supplying me with this valuable information my best thanks are 
due to Dr. Stechow. 
2 Stechow: '07, p. 192; same: '09, p. 21. 
THE JOURNAI. OK EXPERI M ENT At. ZOOLOGY, VOL. 'J, NO. 3. 
