RUGOSA. 207 



bined with tabulce as well. The corallum may be simple or 

 compound, but in the latter case a true ccenenchyma is not 

 present. 



Seeing that there is this striking general likeness between 

 the corallum of the Rugosa and that of the Zoantharia sclero- 

 dermata, and seeing, further, that the only living forms which 

 have any claim to be regarded as Kugose Corals are unques- 

 tionable and undoubted Actinozoa, it would seem that there 

 ought to be little difficulty in deciding as to the systematic 

 position of the Rugosa, unless it were in separating them 

 satisfactorily from the Zoantharia, Of late years, however, 

 an opinion first started by the late Prof. Agassiz has 

 gained considerable acceptance, according to which the 

 Rugosa are to be considered as really belonging to the 

 Hydrozoa. So far as any actual evidence in support of 

 this view goes, it can only be said that we really do know 

 now of living Hydrozoa which do secrete a calcareous skele- 

 ton (Millepora and the Stylasteridce). The skeleton of these 

 forms, however, does not present any special resemblance to 

 that of the Rugosa, and its characters, indeed, are such that 

 its makers till their soft parts were investigated were 

 always referred to the Zoantharia. It is clear, therefore, 

 that the existence of coralligenous Hydrozoa cannot be used 

 as an argument for removing the Rugosa from the Actinozoa. 



On the other hand, the resemblances between the corallum 

 of the Rugosa and that of the Zoantharia sclerodermata are 

 so numerous and so weighty, that it is difficult to imagine 

 that the coralla in the two cases were secreted by different 

 methods, or bore dissimilar relations to the soft parts of the 

 animals producing them. Thus, in both groups alike the 

 simple form of corallum (fig. 98) consists of an outer wall 

 or " theca," enclosing a central space or " visceral chamber," 

 which is ordinarily divided into a series of compartments by 

 vertical partitions or " septa ; " in both alike the " visceral 

 chamber " may be partitioned off into storeys by horizontal 

 plates or " tabulse ; " in both alike the interseptal loculi are 

 liable to be more or less subdivided by " dissepiments ; " 

 and in both alike the axial rod, known as the " columella," ' 

 may be developed. In both groups alike, moreover, the 



