i6o CORALS 



The Order Gymnolaemata is again divided into three 

 Sub-orders : 



Sub-order i. Cyclostomata. 



2. Cheilostomata. 



3. Ctenostomata. 



Of the various groups into which the Class is thus divided, 

 only two Sub-orders, the Cyclostomata and Cheilostomata, 

 provide examples of Polyzoa with calcareous walls. In the 

 others the walls of the zooecia are either horny, mucilaginous, 

 or free from any protective secretion. 



Cyclostomata. — The coral structures formed by the 

 Cyclostomata usually consist of calcareous tubes with a 

 single circular orifice at the terminal extremity. These 

 tubes are usually closely bound together in bundles for the 

 greater part of their course, and in some genera the bundles 

 of tubes become so densely calcified that their tubular 

 nature cannot be determined by superficial examination, 

 although it is indicated b}/ the end which bears the orifice 

 projecting freely on the surface of the zooecium, and it can 

 be readily seen in transverse or longitudinal sections of the 

 main branches of the colonies. 



Crisia. — One of the commonest and most widely dis- 

 tributed of the Cyclostomata is the genus Crisia (Fig. 75). 

 On our own coasts little bushy tufts of Crisia ehurnea are 

 often found attached to the zoophytes and seaweeds that 

 are cast up on the beach after a storm. They are not more 

 than one inch in height, and when seen by the naked eye 

 might be mistaken for the alga Corallina officinalis (see p. 

 207). An examination with a low-power magnifying glass 

 at once reveals their fragile tubular structure, and the large 

 round orifices of the zooecia enable the naturalist at once 

 to separate it from the coralline Algae. 



In the species referred to, the branches are composed of 

 tubular zooecia arranged alternately right and left, and 

 almost entirely adnate, the orifices being only slightly raised 

 from the surface on short tubular projections. 



An important feature of the genus is that at intervals in 

 the course of the branches the hard calcareous structures 



