726 TRANSACTIONS OF SECTION D. 



avicularia is due to a combination of more than one allelomorph we may recognise 

 the possibility that the ancestrulaj of a given species still carry the determinants 

 representing those allelomorphs. In species in which the vicarious avicularia are 

 of normal occurrence there is no difficulty in this hypothesis. In others, of which 

 examples may be found in Schizoporella, the vicarious avicularia make their 

 appearance rarely, in a very small proportion of colonies. In these cases the 

 facts might be accounted for on the hypothesis of the chance recombination of 

 allelomorphs which are ordinarily separated, unless indeed it should prove to be 

 the case that the vicarious avicularia represent a recessive character which is 

 usually prevented from making its appearance by some dominant factor. 



A single series of cases of this kind will not carry conviction, but there are 

 many facts with regard to the distribution of adventitious avicularia that may 

 point in the same direction. We may recur to the fact that the form of these 

 appendages may be eminently characteristic of a whole series of species which 

 from their similarity in other respects are naturally associated in a single genus or 

 family. The most striking instance of this is, perhaps, the genus Bugula, in which we 

 find the avicularium par excellence. The variations of this type of avicularium are 

 comparatively slight and for the most part depend on differences in position with 

 regard to the zooecia and on minor modifications of size, shape, and length of 

 stalk. Both in Bugula and in the allied genus Bicellaria the avicularian 

 characters may be described as relatively constant ; and since they belong to a 

 type that is rarely met with in other genera, they seem to confirm the evidence 

 afforded by other structural features that the species which possess them are 

 related to one another. But even in Bugula, where the avicularia reach the 

 summit of their development, we meet with species or varieties in which these 

 appendages are invariably absent throughout the colony. This may be illustrated 

 by Bugula neritina, a widely distributed species which in the Mediterranean and 

 certain other districts is remarkable for the complete absence of avicularia, 

 although in other structural features it shows a close affinity to other species of 

 Bugula. In Australian and Oriental waters, however, there occur forms which 

 can hardly be distinguished from B. neritina except by the fact that they always 

 possess numerous avicularia of the specialised character that is so distinctive of 

 the genus. It does not matter for our present purpose whether these are to be 

 regarded as a variety of B. 'neritina or not. If the appearance of avicularia may 

 be regarded, on Mendelian principles, as due to the presence of one or more allelo- 

 morphs, it is possible to understand that these may be omitted in certain cases, 

 and that there may thus be a close affinity between two forms one of which 

 differs from the other in what appears at first sight so essential a respect as the 

 complete absence of the avicularia which we are justified in regarding as the most 

 important feature of the genus. 



A second case of the same general nature may also be noticed. In the family 

 CellulariidEB are included a number of delicate erect species which are commonly 

 placed in the genera Caberea, Scrupocellana, Menipea, and Cellularia. The first 

 two of these are distinguished by possessing vibracula as well as avicularia. 

 Menipea is defined as possessing avicularia, but no vibracula ; while Cellularia 

 peac/iii does not possess either kind of appendage. A species known as Amastigia 

 nuda has been placed in a separate genus because of the absence of vibracula and 

 their replacement by avicularia, while in other respects it agrees with Caherea, 

 in which the vibracula reach a development not exceeded by those of any other 

 Cheilostome. Before considering the bearing of these facts we may appropriately 

 consider another instance taken from the same family, although by doing so we 

 are for the moment leaving the question of the avicularia. In the genera 

 Caherea, Scrupocellaria, and Menipea certain species are distinguished by having 

 the free surface of the zooecium protected by a peculiar spine known as the 

 ' scutum,' which is usually flattened and much expanded at its free end, where it 

 overarches the membranous frontal surface in such a way as to cover and 

 presumably to protect it. But in each genus other species are characterised by 

 the complete absence of the scutum, while in others it occurs in varying degrees 

 of reduction. 



