720 TRANSACTIONS OF SECTION D* 



the outside are probably of importance in the respiration of tile animal. The 

 advantages of having avicularia in such a position that they can guard the orifice 

 from which the tentacles are protruded and tjiat of the compensation -sac are too 

 obvious to require detailed discussion. 



The avicularia probably afford little if any protection against the attacks of 

 the larger foes, such as Fishes, Echinids, and Nudibranch Molluscs, which are said 

 to browse on Polyzoa. But there is one group of enemies against which the 

 opercula and the mandibles are probably particularly effective. These are 

 encrusting organisms, including other species of Polyzoa; and indeed the enemies 

 against which a Polyzoon has to provide are probably in a special degree the 

 members of its own class. 



In many Polyzoa which afford large surfaces suitable for the growth of 

 encrusting organisms the older parts of the colony, where the opercula and man- 

 dibles are no longer in working order, do actually harbour large numbers of 

 encrusting Polyzoa, Sponges, Ascidians, and other organisms. These are usually 

 absent in the active parts of the colony nearer the growing margins. In the.se 

 position."? the only animals which obtain a footing are such forms as the Infusorian 

 FolliciiUna, adapted by its minuteness to find a place between the defensive 

 appendages, or such organisms as are attached by means of delicate creeping 

 stolons or rootlets that can find their way between the opercula and mandibles 

 without being damaged by them. A branching species fi.xed by a narrow base 

 may do little harm to a Polyzoon on which it is growing. But the effects of an 

 encrusting species would be different, since the orifices of the colony which is 

 being overgrown would be occluded, and the polypides entirely cut off from the 

 outer world. Although experimental evidence is at present wanting to prove 

 this view, I have little doubt that the avicularia are specially important in 

 preventing the fixation of the larvte of encrusting species. The larva is of course 

 very vulnerable, and it cannot become the founder of an adult colony unless it 

 can find a secluded spot in which it is safe from undue disturbance durino- the 

 critical time of its metaraorpho.sis. The avicularia are well adapted by'^their 

 form for warding off larvie. Those that have the .oo-called 'duck-billed' (U' 

 ' spatulate ' form are in many cases large enough to catch or crush a larva witliout 

 difficulty, while those which liave a mandible terminated by a spike-like projection 

 must be even more destructive to the life of any minute organism which is so 

 imprudent as to stray within their reach. In some of the avicularia belonging to 

 this latter type the mandible is strongly compressed along the greater par" of its 

 length, and may then assume the shape of a knife-blade, with a sharp cutting 

 edge and a thicker back. The blade shuts down into the calcareous rostrum o't' 

 the avicularium in such a way that its action may be compared to that of a pair 

 of scissors. It cannot be doubted that this form of avicularium has a high 

 protective value. 



In some cases the mandibles or the opercula are toothed. The teeth are 

 specially strong in certain species of the genus Sfeffa)ioporella,vfhere the opercula 

 become most formidable weapons. The large development of the occlusor muscles 

 proves that the closure of these opercula must take place with much force. 



The protective value of the avicularia may be illustrated by the distribution 

 of these organs in Refejiora, the species of which usually grow in the form of 

 a calcareous network, with oval meshes or ' fenestrce ' between the branches. 

 These are furnished with an elaborate armature of avicularia, which usually occur 

 in large numbers and in considerable variety of form and size. Some of them 

 are scattered over the front surface, on which alone the zocecia open, while others 

 occur on the more unprotected backs of the branches, where there are no zooecial 

 orifices. To guard against an attack from the rear the margins of the fenestra) 

 are frequently furnished with avicularia, among which some are usually of a 

 specially large size, and are well situated to intercept any larva or adult animal 

 that might attempt to pass through a fenestra. 



A healthy iletepore is usually completely free from encrusting organisms in 

 regions where the avicularia are functional. One of the few exceptions I have 

 noticed to this rule is specially instructive. In this case a small encrusting 



