256 CGELENTERATA. 



As regards the classification of the Actinozoa, no arrangement that 

 has yet been proposed can be regarded as more than provisional, 

 the true affinities of various important fossil groups being still uncer- 

 tain. Usually the Actinozoa have been divided into the four orders 

 of the Zoantharia, Alcyonaria, Cte?iophora, and Rugosa. The first 

 three of these divisions are largely represented by living forms, and 

 may be regarded as essentially natural groups ; but the division of 

 the Rugosa is certainly unnatural, and cannot be retained in its old 

 sense. With our present knowledge — derived wholly from the skel- 

 etal structures — it is not possible to speak positively as to the precise 

 relationships of many of the organisms included in the old group of 

 the Rugosa. There is no good reason for separating certain of the 

 so-called Rugose Corals from existing groups of the Zoantharia 

 Aporosa ; but it would not appear to be expedient, in the meanwhile 

 at any rate, to refer the whole of the Rugosa to the above-mentioned 

 division of the Madreporarians. The three principal groups of the 

 Rugosa — viz., the Cyathophylloids, Zaphrentoids, and Cystiphylloids 

 — are closely connected with one another, and have certain peculiar 

 features of their own. For the reception of these, therefore, it seems 

 to be best, provisionally at any rate, to retain the general name of 

 Rugosa, but to regard the forms in question as constituting a special 

 section of the Madreporarians rather than a distinct order. 



As regards their general distribution in space, all the recent Actin- 

 ozoa are inhabitants of the sea, and there is no reason to suppose that 

 any of the fossil forms were other than marine. The coralligenous 

 forms, generally spoken of as " Corals," are partly inhabitants of 

 deep water, partly shallow-water types ; and the latter are well known 

 as giving rise in warm seas, and under suitable conditions, to those 

 great aggregations of coral which are known as " coral-reefs." It is 

 not necessary to consider here the general phenomena of existing 

 coral-reefs, or to enter into a discussion of the vexed question as to 

 the laws and conditions under which the various forms of these 

 structures are produced. A few words may be said, however, as to 

 the nature of the materials which actually form a recent coral-reef, 

 as this is a point which has important geological bearings. In the 

 first place, though Corals properly so called are the principal agents 

 concerned in the construction of a coral-reef, they are not the only 

 ones, a very important part in the formation of the reef being played 

 by other organisms which secrete a calcareous skeleton. Thus, the 

 materials composing a coral-reef are largely the result of the accumu- 

 lation of the calcareous skeletons of animals other than corals, and 

 particularly of the Hydrocorallines and the Molluscs ; while the cal- 

 careous Algce (Nullipores) take an important part in the formation 

 of some reefs. Again, it is only a portion — chiefly the outer edge — 

 of a modern coral-reef which is actually composed of living corals, 



