492 



NATURE 



[September 17, 1908 



encrusting organisms, and at the same time of the success 

 with which certain animals have ignored the defensive 

 works that are effective against ordinary foes. This is 

 the common Flustra joliacea of our own shores, in which, 

 although avicularia are present, the defence is provided 

 largel)' by the numerous stiff spines which make its surface 

 irregular, and thus unsuitable for the growth of an encrust- 

 ing organism. But certain delicate Polyzoa, such as 

 trisia and Scrupocellaria, which are attached by fine root- 

 lets, 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 margarica) of a 

 -similar mode of growth is also known to occur on healthy 

 colonies of Flustra joliacea. 



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 

 Caberea, in which vibracula occur on the backs of the 

 ■branches, usually have a much laxer mode of growth than 

 Bugula, 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 membrane, 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 Star- 

 fishes can devour Molluscs to the shells of which thcv 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, .'\lthough the enemies to be 

 guarded against are not the same in the Polyzoa, there 

 may, none the less, be an advantage in having the cal- 

 careous parts covered with an organic membrane. The 

 species which are especially liable to the attacks of Folli- 

 culina appear to be those in which the calcareous parts 

 are but little protected, as in Cyclostomes such as 

 I.ichenopora ; while this Infusorian readily establishes itself 

 on dead parts of Cheilostomes 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 lo 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 zooecia. There are many cases, such as Schizo- 

 porella Cecilii, MticroneUa ventricosa, and a number of 

 others, in which the zooecia of a species devoid of avicularia 

 are very convex in their external shape. The conjunction 

 of a succession of convex zooecia is probably important in 

 preventing the encroachments 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 joliacea, which we have 

 already noticed, where the protection appears to depend 

 largely on the development of spines. The irregular 

 surface of many Cyclostomes, which is due to the projec- 

 tion of the free ends of the zooecia, is probablv similarlv 

 effective in preventing overgrowth by foreign organisms. 



In the vibraculum the p.irt that corresponds with the 

 mandible of the avicularium has been prolonged into a 

 thread-like structure, the "seta," which is moved bv 

 muscles corresponding with those of the avicularium. 



The seta; of Caberea are very large, and thev close into 

 oblique grooves which run along the back of the branch. 

 The protective value of these setae is well shown in a 

 specimen I have observed from Torres Straits, in which 

 .T minute encrusting Cheilostome has formed a single row 

 of zooecia 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 



NO. 2029, VOL. 78] 



purposes besides the protection against living foes. They 

 no doubt serve to brush away foreign particles which 

 might otherwise settle on the surface of the colony and 

 block up the orifices. This function has been suggested 

 for the vibracula of the so-called Selenariidas, a group of 

 forms which agree in having a zoarium of a discoidal 

 or inverted saucer-like shape. The colony is believed to 

 rest freely on the bottom, on the edge of its concave base, 

 though 1 have some evidence that it may be attached to 

 the ooze by means of very delicate, flexible, rooting pro- 

 cesses. Some at least of these Selenariiform species occur 

 in situations where the ground is covered by Globigerina 

 ooze, the settlement of which on the convex surface bear- 

 ing the orifices is probably prevented by the vibracula. 

 It is now generally admitted that his type of colony has 

 been independently acquired in several cases, the so-called 

 family being, in fact, an entirely unnatural assemblage 

 of genera. It may be worth while to point out in passing 

 that I have noticed in several cases that the Selenariiform 

 colony commences its existence on a Foraminiferan shell 

 or other minute object, in the absence of larger surfaces 

 on which fixation can be effected, and that the character- 

 istic discoidal form is due to the growth of the circular 

 edge of the colony beyond this initial supporting base. 



But my object in introducing this group of Cheilostomes 

 at the present point is to direct attention to the relatively 

 enormous size which is reached by the setai of the vibracula 

 of some of the species, a size which is so great that it 

 has even been supposed that these appendages are used 

 as oar-like organs of locomotion. In a specimen of 

 Selcnaria hexagonalis, from South .\ustralia, in the Museum 

 of Zoology at Cambridge, the seta; have been colonised 

 by a minute Cheilostome belonging to the genus Eucratea. 

 It might be said that in this case the setae have almost 

 over-reached themselves, since they have become so large 

 and powerful that another species is minute enough to 

 find a home on the protective mechanism itself. 



Having thus dealt with the probable functions of the 

 avicularia and vibracula, we mav now return to the con- 

 sideration of the forms assumed by these appendages and 

 of their distribution in the colony. The protective function 

 which they appear to possess prepares us for finding, as 

 is actually the case, that they are modified in an extra- 

 ordinary number of directions. But although they occur, 

 in one form or another, in the majority of Cheilostomes, 

 they may be completely absent in an entire genus, in 

 certain species of a genus, in certain varieties of a species, 

 or in individual colonies of species which normallv possess 

 them. They are often wanting on some of the zooecia, 

 though present on most of the zooecia, of a colony ; and 

 they may vary to a considerable extent in the position 

 they assume on the zocecium. Not only are they thus 

 variable in their occurrence, but they show equally striking 

 differences in their individual characters. They may be 

 all of one kind in a single species, or two or more kinds 

 may occur distinguished by size, by the shape of the 

 rostrum and mandible, or in other ways. We thus come 

 to the consideration of the question how far these append- 

 ages can be used in the discrimination of species. 



The characters on which species are founded in a group 

 of colonial animals like the Polyzoa obviously differ in 

 certain respects from those which are used in distinguish- 

 ing species in organisms that lead a solitary e.xistence. 

 In the colonial forms we are concerned partly with the 

 mode of association of the individual units, partly with the 

 manifestations of dimorphism or polvmorphism shown by 

 those units and partly by the features of the individuals 

 themselves. Among the Cheilostomatous Polyzoa the 

 dimorphism or even polymorphism of the individual, due 

 to the presence in the colony of avicularia and vibracula, 

 is of special importance. 



\\'hile the characters of the avicularia ha%'e accordingly 

 long been used by systematists for distinguishing species, 

 no one — so far as I am aware — has hitherto suggested 

 any hypothesis which helps us to form a reasonable con- 

 ception of the significance of the innumerable modifications 

 undergone by these organs ; nor do I think that the 

 problem has ever been fairlv stated. 



The difficulty of understanding the evolutionarv signifi- 

 cance of the avicularia arises in part from the fact that 

 the occurrence and distribution of these structures appear 



