PRESIDENTIAL ADDRESS. 723 



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 

 Selenariidse, 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 I have some evidence that it may be 

 attached to the ooze by means of very delicate, flexible, rooting processes. 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 

 bearing the orifices is probably prevented by the vibracula. It is now generally 

 admitted that this 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 characteristic 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 call attention to the relatively enormous size which is reached by the setae 

 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 Selenaria he.vagonalis, from South Australia, in the Museum of 

 Zoology at Cambridge, the setae have been colonised by a minute Cheilostome 

 belonging to the genus Eucratea, It might be said that in this case the setae 

 have almost overreached 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 may now return to the consideration 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 extraordinary number of directions. But although they 

 occur, in one form or another, in the majority of Cheilostomes, they may be com- 

 pletely absent in an entire genus, in certain species of a genus, in certain varieties 

 of a species, or in individual colonies of species which normally possess them. 

 They are often wanting on some of the zooecia, though present on most of the 

 zocecia, 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 dis- 

 tinguished 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 appendages 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 dis- 

 tinguishing species in organisms that lead a solitary existence. In the colonial 

 forms we are concerned partly with the mode of association of the individual 

 units, partly with the manifestations of dimorphism or polymorphism 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. 



While the characters of the avicularia have accordingly long been used by sys- 

 tematists for distinguishing species, no one — so far as I am aware — has hitherto 

 suggested any hypothesis which helps us to form a reasonable conception of the 

 significance of the innumerable modifications undergone by these organs; nor 

 do I think that the problem has ever been fairly stated. 



The difficulty of understanding the evolutionary significance of the avicularia 



3 A2 



