721 TRANSACTIONS OF SECTION D. 



arises in part from the fact that the occurrence and distribution of these structures 

 appear in many cases to give but slight indications of affinities. It cannot, for 

 instance, he assumed, without further evidence, that two species possessing an 

 identical type of avicularium are nearly related. The complete absence of avicu- 

 laria in a particular species is no sufficient reason for removing that species from 

 an assemblage of forms in which avicularia are always present. And, lastly, there 

 may be good grounds for believing that two forms with entirely different types of 

 avicularia are closely related, and in some cases may even belong to the same 



species. 



The result of a comparative study of the Cheilostomata leads in fact to the 

 conclusion that although certain genera or species are characterised by the posses- 

 aion of one or more definite types of avicularium or vibraculum, other genera or 

 species show no such constancy in this respect. The occurrence of the same type 

 of avicularian appendage in the species of widely separated genera and the diver- 

 sity of type of avicularium withia the limits of a single genus or species render it 

 most difficult to frame any theory that will account for the facts. Are we to 

 assume that a given type of avicularium has been evolved independently in a 

 number of cases, or must we suppose that species with that type have inherited 

 it from a common ancestor ? If the latter hypothesis be the correct one, we seem 

 to be led to the conclusion that the ancestral Cheilostomes were provided with 

 most of the types of avicularia that actually occur in existing species, many of 

 which have lost one or more of those types. 



In tryiog to arrive at some conclusion with regard to these points we may 

 notice in the first instance one fact which stands out with great distinctness — 

 namely, that, whatever the modifications of the avicularium may be, the mandible 

 is usually either acutely pointed at its free end or rounded and spatulate at its 

 termination. The difference may at first sight appear unimportant, but I am 

 inclined to believe that it is an indication which may lead us to results of great 

 significance. 



Thouo-li it may be going too far to assert that all avicularia belong to one of 

 these two types, there is usually no difficulty in recognising either the pointed or 

 the rounded character in every avicularium present on a colony. The distinction 

 may be observed by inspecting the form of the rostrum in a dry preparation of a 

 part of the zoarium, but it is seen with more certainty when the mandibles have 

 been isolated and are examined in Canada balsam. So striking is the difference 

 that the inquiry naturally suggests itself whether there is any indication of the 

 evolutionary meaning of the two kinds of avicularium. It appears to me probable 

 that a condition which is characteristic of the existing genus Steganoporella may 

 furnish the answer to this question. In this genus avicularia are typically absent, 

 hut in each species the zorecia are of two kinds, distinguished by differences in the 

 shape and structure of the opercula and orifices. The anatomy of the zooecia is 

 known in but few cases, but in those that have been observed both kinds of 

 zocecla possess polypides. In one division of Ster/anojyorel/a the more differentiated 

 zooecia show some it semblances to the pointed type of avicularium, while in a 

 second division they more nearly resemble rounded avicularia. I am inclined to 

 believe that these conditions correspond respectively with the two kinds of 

 differentiated avicularia of other Cheilostomes. 



The avicularia most commonly met with occur as appendages of the ordinary 

 zooecia, which alone constitute the main framework of the colony. But in addi- 

 tion to these, the 'adventitious ' avicularia of Busk, we find, although less com- 

 monly, another kind known as the ' vicarious ' avicularium, from the fact that it 

 occupies the place of an ordinary zooecium, with which it agrees more or less 

 closely in point of size. Its mandible is usually of the rounded type, appropriately 

 referred to as ' duckbill-like,' and is readily seen to represent the operculum of 

 an ordinary zooecium. Compared with this the mandible and the orifice which it 

 closes are greatly enlarged, while the occlusor muscles have become correspond- 

 ingly increased in size. The polypide is generally absent in the vicatious 

 avicularium. 



Pointed avicularia of the vicarious type occur normally in the species of 



