58 PEOCEEDINGS OF THE 



The definition of these groups will, of course, be arbitrary ; tbey 

 will often shade off into one another, and tlie precise position of 

 any given example may be difiicult to determine. 



I propose to-day to describe a few more or less familiar 

 instances of Commensal and Symbiotic forms. The term Com- 

 mensal I apply only to those groups in which one organism, the 

 self-invited guest, derives advantage from its host without being 

 injurious to it. 



A deep-water hexactinellid sponge, GrateromorpJia Meieri, 

 shaped like a wine-glass, has its main body three or four inches 

 long, with its walls exceedingly loose, owing to the great develop- 

 ment of its canal-system. In almost every example of this 

 sponge may be found a remarkable worm, Syllis ramosa, which, I 

 believe, was first described in the Journal of this Society by 

 Prof. M'Intosh*. "When young, the embryo worm seeks shelter 

 in the canal-system of the sponge, where it is protected against 

 its numerous enemies, since the spicules, and sometimes the flesh 

 also, of sponges is objectionable to most animals. The water- 

 currents in the sponge bring to the worm the necessary food and 

 oxygen, so that there is no occasion for it to leave its home. By 

 a continued process of budding the worm acquires a much- 

 branched form, the branches extending into the various canals 

 of the sponge and projecting into the interior of its cup. The 

 form of the worm is quite incompatible with a free life, but 

 admirably fits it for its residence. 



One of the Polychsetous worms, Eunice phylocorallia, has been 

 recently described by Miss Buchanan ; it has only been found 

 associated with a coral, Lopliohelia prolifera. The presence of 

 the worm in one place amongst the branches of the coral induces 

 an extension of the coenenchyma around it, forming a tube in 

 which the worm dwells in safety, deriving additional protection 

 by the surrounding hard branches of the coral with their stinging 

 polypes. 



Crustacea, like the Annelida, form the favourite prey of many 

 animals ; we accordingly often find that they seek protection by 

 associating themselves with some other organism. Thus, in the 

 well-known instance described by Semper, we find a small crab, 

 Saplocarcinus marsiipialis, when quite young taking up its 

 abode in the fork of the branches of a coral, either a species of 

 Seriatopora or Pocillopora. The coral is stimulated to increased 

 growth and forms a gall-like structure ; in a cleft at the bottom 

 of this the crab dwells in security, feeding on such animals as 

 may wander into the space above, which is enclosed by the over- 

 arching branches of the coral. 



The numerous species of Pergoma (one of the Cirripedia) find 

 protection by developing each in its own particular coral. 



Coronula^ another member of the same group of Crustacea, is 

 fixed in a very beautiful way to the skin of certain species of 

 whale, gaining thereby free locomotion. 



* Journ, Linn. Soc, Zool. vol. xiv. p. 720. 



