208 Mr. A. Alcock on a case of Commensalism hetween a 



sponges, and of Hydranthea viargarica found by the Rev. T. 

 Hincks only on Flustra. Even the cases of Lar sabellarum, 

 found by Gosse, as reported by Alhnan, only on the tubes of 

 Sabella, and of Stylactis vermicola found by the ' Challenger ' 

 only upon a bathybial annelid (Allman, ' Challenger ' Hy- 

 droida, part ii. p. 2, pi. i. fig. 2), may come under the head 

 of accidental association, though they far more probably are 

 examples of a definitely established symbiosis. 



Among the best of the cases of undoubted commensalism, 

 in which one of the associates is a Gymnoblastic Hydroid, 

 are those discovered by Professor Ilfeckel {' Cliallenger ' 

 Deep-sea Keratosa, pp. 75-81, pi. ii. figs. 5, 6, and 7, pi. iv. 

 fig. 4), of Stylactis [StylactcUa) spongicola and ahyssicola, and 

 Eudendrium'^ sp., always found symbiotic with certain deep- 

 sea horny sponges. Here the ramifying hydrorhiza of the 

 polyp, which is greatly developed, affords by its chitinous 

 perisarc a solid supporting framework for the sponge, and 

 determines the form of the latter. The trophosome, on the 

 other hand, is represented by significantly small hydranths. 



Another instance of mutual relations almost as intimate is 

 that reported by Korotncff (Zeitschr. fiir wiss. Zool. Bd. xlv. 

 p. 486, Taf. xxiii. figs. 18-22), of a Tuhularia [T. parasitica) 

 living with a Gorgonia^ the latter having no axis of its own, 

 but using the stem of the Tidjidaria for a support. 



Professor Allman, in his beautiful Monograph, quotes 

 several cases that can hardly be regarded but as exemplifying 

 definite associations for mutual benefit. He himself found 

 Perigonimus minutus entirely confined to the living shells of 

 a gastropod mollusk {Turritella communis), the polyp- 

 colonies forming a fringe round -the operculum of the mollusk 

 in all of about thirty shells dredged. He also quotes the 

 records of other observers, of which the two most remarkable 

 are that of Canon Norman (of Merona coimucopice found only 

 on living shells of Astarte sulcata and Dentalium entalis from 

 80 to 100 fathoms) and that of Professor Gegenbaur (of 

 Campaniclava cleodoroi confined to living shells of the pelagic 

 Cleodora iricusindata in thirty-two out of forty specimens of 

 the latter examined). 



In cases where a hydroid allies itself with a locomotive 

 animal the advantages that the polyps derive from the partner- 

 ship are very clear ; for, as previous observers have pointed 

 out, the polyps, instead of being entirely dependent on chance 

 movements of the sea for uncertain supplies of food and air 

 (as when attached to fixed objects), or for uncertain driftings 

 towards food (as when attached to floating bodies), are rapidly 



