STRUCTURE AND CLASSIFICATION 19 



The Polyzoa never possess nematocysts. Their food 

 is obtained by the action of currents of water pro- 

 duced by the cihary action of the cells that cover the 

 tentacles. 



Of the animal corals that do not bear polyps there are 

 only two groups, and neither of these have many repre- 

 sentatives. 



The living substance of the Foraminifera is not divided 

 up into a number of cell units, but is a continuous mass or 

 plexus of the vital stuff — protoplasm. There is no mouth, 

 no body cavity, and no tentacles, but at the periphery the 

 protoplasm spreads out into a complex web of filaments 

 which can capture and digest small organisms that come in 

 contact with it. The corallum is formed of a number of 

 adjacent chambers which are perforated by an immense 

 number of minute pores — the foramina. 



There are only two or three sponges which can be called 

 Poriferan corals, and these will be described in a later chapter. 

 But, for comparison with other groups, it may be said here 

 that the Porifera are multicellular animals — without any of 

 the characters of polyps — which obtain their food by main- 

 taining a constant flow of water through an elaborate system 

 of canals and spaces in their body, certain cells of which 

 have the power of catching and digesting such organisms as 

 are nutritious. 



The Plant corals all .belong to that division of the 

 Vegetable Kingdom which is known as the Algae. Most of 

 the Algae with which we are familiar are soft and flexible, 

 but two of the classes included in that division, namely, the 

 Rhodophyceae or Red Seaweeds and the Chlorophyceae or 

 Green Seaweeds, include genera which secrete a sufficient 

 amount of calcareous matter to render them hard and 

 resistant. As these coral Algae possess no mouths, holes, or 

 cavities that can be seen except with a high power of 

 the microscope, they were grouped together by the older 

 writers under the common name of " Nullipores," a name 

 which has now generally fallen into disuse. 



The classification of corals adopted in this book may be 

 expressed in a tabular form as follows : 



