722 TRANSACTIONS OP SECTION D. 



the succeSsAvith which certain animals hd,ve ignored the defensive works that aie 

 effective against ordinary foes. This is the common Flustra foliacea of our own 

 shores, in which, although avicularia are present, the defence is provided largely 

 by the numerous stiff spines which make its surface irregular, and thus unsuitable 

 for the growth of an encrusting organism. But certain delicate Polyzoa, such as 

 Crista and Scrupocellaria, which are attached by fine rootlets, flourish on this 

 species, their rooting processes being able to adapt themselves to the irregularities 

 of the surface, and to escape the closure of the opercula and mandibles. A 

 Gymnoblastic Hydroid {Hydranthea margavica) of a similar mode of growth is 

 also known to occur on healthy colonies of Flustra foliacea. 



In a large number of erect Polyzoa the colony, or zoarium, assumes the form 

 of a small branching tree-like growth in which, as in Retepora, the zocecia open 

 on one surface only of the branches. The opposite surface is often devoid of any 

 armature of avicularia or vibracula, a fact which at first sight seems opposed to 

 the view that these structures are protective. But I think that in some of these 

 cases the form of the zoarium affords an answer to this objection, since the 

 branches are so crowded that the avicularia of the front surface of one branch are 

 probably quite capable of affording protection to the backs of the nearest 

 branches. It may be noted that Scrupocellaria and Caherea, in which vibracula 

 occur on the backs of the branches, usually have a much laxer mode of growth 

 than Buyula, in which the back is unprotected. 



In some other erect species there are no avicularia at all. But here we often 

 find, as in Euthyris, that the whole of the frond is covered by an organic mem- 

 brane, the ' epitheca,' which invests the calcareous parts ; and it seems to me 

 probable that this epithecal layer is itself protective. Schiemenz has shown that 

 it is an advantage to certain Molluscs to have an internal shell, since Starfishes 

 can devour Molluscs to the sheila of which they can attach their tube-feet, while 

 they can obtain no hold on the slimy surface of a Mollusc which has covered its 

 shell by part of its soft tissues. Although the enemies to be guarded against are 

 not the same in the Polyzoa, there may none the less be an advantage in having 

 the calcareous parts covered with an organic membrane. The species which are 

 especially liable to the attacks of Folliculina appear to be those in which the 

 calcareous parts are but little protected, as iu Cyclostomes such as Lichenopora ; 

 while this Infusorian readily establishes itself on dead parts of Chcilostomes which 

 have lost the epitheca that covers their active regions. The encrusting species of 

 Polyzoa doubtless prefer a hard calcareous surface on which to grow to a soft 

 yielding membranous surface. 



As a further factor with which the absence of avicularia may be correlated 

 may be mentioned the shape of the individual zocecia. There are many cases, 

 such as Schizoporella Cecilii, Mucrotiella ventricosa, and a number of others, in 

 which the zocecia of a species devoid of avicularia are very convex in their 

 external shape. The conjunction of a succession of convex zocecia is probably 

 important in preventing the enoroachments of encrusting species, which more 

 easily adapt themselves to a level surface than to one which is strikingly uneven 

 or irregular. This is analogous to the case of Flustra foliacea, which we have 

 already noticed, where the protection appears to depend largely on the develop- 

 ment of spines. The irregular surface of many Cyclostomes, which is due to the 

 projection of the free ends of the zocecia, is probably similarly effective in preventing 

 overgrowth by foreign organisms. 



In the vibraculuiu the part that corresponds with the mandible of the 

 avicularium has been prolonged into a thread-like structure, the ' seta,' which is 

 moved by muscles corresponding with those of the avicularium. 



The setae of Caherea are very large, and they close into oblique grooves which 

 run along the back of the branch. The protective value of these setas is well 

 shown in a specimen I have observed from Torres Straits, in which a minute 

 encrusting Cheilostome has formed a single row of zocecia along the region between- 

 two of the vibracular grooves, but has not extended into any part where it would 

 be subject to injury by the movements of the setae. 



The vibracula are, however, probably used for other purposes besides the^ 



