ACTINOZOA 163 



Corallium^ by the fusion of spicules in the mesogloea, in Heliopora the 

 skeleton is secreted by a layer of ectodermal cells and not composed 

 of spicules. In Heliopora (Fig. 135) there are on the surface of the 

 colony larger pits (thecae) occupied by the polyps and smaller pits 



Fig. 135. Diagrammatic section through the edge of a colony of Heliopora. 

 After Kukenthal. The skeleton is shown as deep black, the ectoderm and 

 endoderm as lines and the mesogloea by stippling, pol. polyp ; sol. network 

 of solenia parallel to the surface of the colony; tub. vertical tubules arising 

 from this network; th. theca. 



which lodge tubular processes of the network of solenia: the same 

 skeletal characters also occur in the fossil Heliolites which closely 

 resembles it and was a dominant type in Palaeozoic coral reefs. Tubi- 

 pora too has a Palaeozoic representative in Syringopora.^ 



Order ZOANTHARIA 

 Actinozoa with mesenteries varying greatly in number, typically 

 arranged in pairs, the longitudinal muscles of which face each other, 

 except in the case of two opposite pairs, the directives , in which the 

 muscles are on opposite sides; tentacles usually simple, six or some 

 multiple of six in number; mesenteric filaments trefoil-shaped in 

 section ; stomodaeum with two ciliated grooves ; typically a calcareous 

 exoskeleton, but this may be entirely absent. 



The coelenterate animals which are included in this group fall into 

 two apparently different categories, the sea anemones, which are 

 usually single individuals and never possess any kind of skeleton, and 

 the madreporarian corals, which are usually colonial animals and 

 always have an ectodermal exoskeleton. The polyps, however, may 

 all be referred to the same type of structure, and the presence or 

 absence of a skeleton or of the colonial habit are matters of secondary 

 importance compared with this. 



^ The relationship between these recent alcyonarians and the Palaeozoic 

 corals is denied by some authors. 



