PROTOZOA. 163 



3. Polyps. — There are two groups of Polyps: — 



1. Actinoid Polyps, illustrated in figs. 147, 148, and. all ordinary 

 corals. The rays of the polyps are of variable number, and naked 

 (not fringed). 



The coral is secreted within the polyps, as other animals secrete 

 their bones. It is internal, and not external. It is usually covered with 

 radiate cells, each of which corresponds to a separate polyp in the 

 group. The rays of a cell correspond to fleshy partitions in the 

 interior of the polyp, and the cavity of a cell is the space occupied 

 by the stomach and visceral cavity : it is not, therefore, a cavity 

 into which the polyp retreats ; it is the inside of the polyp itself. 

 The material is carbonate of lime (limestone) ; and it is taken 

 by the polyp from the water in which it lives, or from the food 

 it eats. 



2. Alcyonoid Polyps, illustrated in fig. 149, and the Gorgonia 

 and Alcyonium corals. The rays of the polyps are eight in number, 

 and fringed. The figure represents a part of a branch of a Gor- 

 gonia (Sea-Fan), with one of the polyps expanded. The branch 

 consists of a black horny axis and a fragile crust. The crust is partly 

 calcareous, and consists of the united polyps ; the axis of horn is 

 secreted by the inner surface of the crust. The Precious Coral used 

 in jewelry comes from the shores of Sicily and Southern Italy, and 

 belongs to this Alcyonoid division. It is related to the Gorgonias, 

 but the axis is red and stony (calcareous) instead of being horny, 

 and this stony axis is the coral so highly esteemed. 



Among the Actinoid Polyps there are the following groups, — ex- 

 clusive of those that do not secrete coral : — 



1. The Actinia tribe. — The number of rays a multiple of six. It includes the 

 Astraea family, Oculina family, Fungia family, Caryophyllia family, Madre- 

 pora family, Porites family. 



2. The Cyathophyllum tribe. — The number of rays a multiple of four. It 

 eontains only Palaeozoic corals, replacing in that era the Astraeoid corals of later 

 eras. The animals were probably like the Actinia in fig. 147. 



V. Protozoans. 



The groups of Protozoans of special interest to the geologist are 

 two: — 



1. Rhizopods, or Foraminifers. — Species mostly microscopic, 

 forming calcareous shells. These shells, with few exceptions, 

 are very minute, — many times smaller than the head of a pin ; 

 and yet they have contributed largely to the formation of lime- 

 stone strata. They consist of one or more cells ; and the com- 



