t'ftfisibENTiAL AobRfisS. 7 19 



rrliich has Oilce been-constLtuted at tlie gtowing margin of the colony does not, 

 as a rulej possess the power of giving rise to new zooecia, although it commonly 

 has the faculty of producing sexual cells from which free larvie may develop. 



In the majority of the species of Cheilostomata many of the individuals of the 

 colony have the form of the so-called avicularia. An avicularium is characterised 

 by possessing a chitinous -'mandible,' which can be closed with great force by 

 strono- occlusor muscles, the organ being thus essentially of a prehensile nature. 

 There can be little doubt that the mandible is a modification of the chitinous 

 operculum which closes the orifice of the tentacle-sheath in Cheilostomata. It 

 thus follows that avicularia are restricted to this particular division ^ of the 

 Polyzoa. In the avicularium the operculum has become relatively and often 

 absolutely enlarged, and its muscles have become more powerful than those of the 

 unmodified zocecia. The internal viscera have, as a rule, disappeared, and there 

 are thus neither tentacles uor alimentary canal. The body-wall, or zocecium, has 

 become a case which contains the muscles, while part of it has been prolonged 

 into a beak-like structure or ' rostrum,' which, with the chitinous mandible, 

 constitutes the prehensile mechanism. 



In Bwyula and its allies the avicularium has the form to which its name refers, 

 and has a striking resemblance to the head of a bird like an eagle or vulture. 

 This resemblance is due, not only to the general form of the structure, but also 

 to the hooked and beak-like shape of its rostrum and to the narrow neck by 

 which it is connected with the zooecium on which it is borne. The avicularia 

 of Bugula have considerable powers of movement, and in the living condition 

 they may be seen to bend backwards and forwards on their fiexible neck, their 

 range of action being thus considerably enlarged. The mandible is ordinarily 

 held wide open, but it closes with great force when some foreign object is placed 

 between the jaws. An avicularium which has in this way seized a small worm, 

 for instance, is known to be able to retain its capture for many hours, in some 

 cases for more than an entire day. 



In the majority of Cheilostomata the avicularia are, however, not stalked. 

 They occur scattered over the colony in a considerable variety of positions, and 

 usually appear as appendages rigidly connected with the walls of the zocecia. 



More than one attempt has been made to explain the functions of the 

 avicularia. The distribution of these organs indicates, I think, that the simplest 

 and most obvious explanation is the correct one— namely, that, like the 

 pedicellariae of Echinoderms, they are defensive organs. The ordinary unmodified 

 opercula probably have the same function in many cases ; and if account be taken 

 of the fact that the avicularium is morphologically a modified zooecium it becomes 

 easy to understand that the defensive office of the opercula has been made more 

 etticient in specially modified zooecia which concentrate their energies on this one 

 function alone. 



A casual inspection of a number of Cheilostomes taken almost at random 

 reveals the fact that the avicularia are specially common in the immediate neigh- 

 bourhood of the orifice of the tentacle -sheath and of that of the ' compensation- 

 sac' 



This is a thin-walled cavity which in a considerable proportion of the 

 Cheilostomes opens to the exterior at the proximal border of the operculum. Its 

 principal function is to permit protrusion and retraction of the polypide to take 

 place, since in a zocecium with completely rigid walls the act of protrusion could 

 not occur if the temporary removal of structures of considerable size were not 

 compensated for by the admission of water into the space included by the 

 ritrid body-wall. At each movement of protrusion, therefore, a volume of water 

 corresponding with that of the protruded organs is admitted into the compensation- 

 sac, the dilatation of which, by means of radiating muscle-fibres, is the cause of 

 the protrusion ; and is again evacuated when the polypide is retracted. These 

 alternate actions of filling and emptying the compensation-sac with water from 



' It m^y be noted that Palteontologists have described structures which they 

 have regarded as avicularia in Polyzoa which do not belong to the Cheilostomata, 



