Presidential address. 72 1 



Cbeiloatome coloDy is growing near the edge of the Retepora frond. The 

 primary individual or aucestrula of the encrusting colouy is readily distinguish- 

 able, and its position shows that the larva from which it was formed must have 

 attached itself to the growing margin of the Retepore, a region in which the 

 avicularia were not fully formed. Having thus established itself, the colony has 

 succeeded in invading a small region of the adjacent parts where the zooecia are 

 still vigorous and healthy. A dead Retepora, on the contrary, forms a substratum 

 which is well adapted for the growth of various organisms, such as other Polyzoa, 

 Sponges, Hydroids, Compound Ascidians, and Foraminifera. 



Although the avicularia are thus effective in preventing the overgrowth of 

 the colony by most of the organisms that might otherwise settle there, there are 

 one or two animals of suitable habit which have succeeded in establishing them- 

 selves in the very midst of the defensive works. In species of Retepora from 

 the Malay Archipelago ' I find that a considerable proportion of the colonies are 

 infested by a Gymnoblastic Hydroid of SijiKoryne-Yike appearance. The associa- 

 tion of this with its host is so intimate that the hydrorhiza becomes completely 

 included in tunnels formed in the calcareous mass of the Polyzoon, where it is, of 

 course, safe from the avicularia. These tunnels, the walls of which are secreted 

 by the Polyzoon, open to the exterior by tubular apertures situated on the margins 

 of the fenestras and on other parts ; and they are so definite in their appearance, 

 and often so regularly arranged, that it might be difficult to believe that they 

 were not a normal feature of the Retepora were it not possible to demonstrate 

 their relation to the Hydroid.^ 



There is one other organism which has a definite relation to colonies of 

 Retepora in Malay waters. This is Loxosoma, a stalked Polyzoon which leads 

 a practically solitary life owing to the fact that its buds break off as soon as they 

 have reached maturity. The Loxosoma no doubt succeeds in enjoying a secure 

 existence, even though it is surrounded by avicularia, owing partly to its stalked 

 form and partly to its minute size. It is commonly found in considerable 

 numbers, and often attaches itself in such a way that it projects into one of the 

 fenestrae, where it is as far as possible removed from the dangerous neighbour- 

 hood of the avicularia. 



We have thus seen that, while the species of Retepora are adequately pro- 

 tected against many encrusting or epizoic organisms, there are one or two animals 

 that have succeeded in evading the attacks of the avicularia, Avhich, it must be 

 remembered, are rigidly attached to the colony, and cannot go in search of any 

 enemy that keeps out of their way. The eflicient avicularian protection may 

 well be responsible for the fact that Retepora is a common and v/idely distributed 

 genus, flourishing in both shallow and deep water. Not only is it found in large 

 numbers in the most diverse locahties, but it has differentiated itself into a large 

 number of species, among which avicularia occur in great profusion and in a 

 great variety of forms. But so soon as the avicularia cease to be active we find 

 that numerous organisms settle on the unprote^gted branches ; and a dead colony 

 of Retepora is accordingly usually found to be invaded by numbers of other 

 animals. 



One other familiar case may be mentioned illustrative of the means by which 

 a Polyzoon may be protected from encrusting organisms, and at the same time of 



' The greater number of the facts referred to in this Address have been observed 

 during my study of the Polyzoa collected during the Slboya expedition. 



''■ It may be noted, as has already been done by Alcock {Arm. 3Jag. Xat. Hist., 

 ser. 6, X. 1892, p. 207), that many other cases are known in which there is an 

 association between a Gymnoblastic Hydroid and some other animal. The interest- 

 ing case of the association of a Gymnoblastic Hydroid {Stylactis) with a fish 

 (Minous) described by Alcock has also been described, more recently, by Franz and 

 Stechow (Zool. Anzeiger, xxxii. 190S, p. 752). Another case of the association of a 

 Coelenterate with a Polyzoon has been recorded by Haswell and by Kirkpatrick, 

 who have called attention to the occurrence of a small ' Actinid ' which forms 

 definite cavities in a massive calcareous Cheilostome from Australian waters. There 

 is in this case no satisfactory evidence to show what the Cceleutcrate really is. 



1908. 3 A 



