130 



ANIMAL KINGDOM. 



a', Sertularia abietina ; 6, b', 

 rosacea. 



Hydroids. Many of them make corah, and hence are common as 

 fossils. Fig. 141 represents a Hydra enlarged, with a young one 

 budded out from its side. Some species of the group, — those of the 

 Sertularia tribe, — form delicate membranous corals, such as are rep- 

 resented in Fig. 1 G9 B, in which each notch on the little branchlets 

 corresponds to the cup-shaped cell from which an animal protrudes 



its flower-shaped head, (a is the 

 Sertularia abietina ; b, S. rosacea ; 

 and a', b 1 , portions of branches 

 enlarged). The interior cavities 

 of each animal communicate free- 

 ly with the tube in the stem ; and 

 in this they differ from Bryozoans, 

 whose groups have no tubular 

 axis. The ancient Graptolites 

 (some of which are represented on 

 page 187) are supposed to have 

 been of this nature. Others se- 

 crete calcareous corals of large 

 size, and are called Millepores (be- 

 cause the minute cells from which 

 the animals protrude are like pin-punctures in size, and very numerous 

 over the surface of the coral). The Millepores are common in the 

 West Indies and other coral seas. The minute animals of a Millepore 

 have nearly the form represented in Fig. 142, p. 117, which represents 

 a species of another genus, called Syncoryne. 



There are hence stony corals made by Polyps, by Hydroid Acalephs, 

 and by Bryozoan Mollusks. 



3. Polyps. — There are two groups of coral-making polyps : — 



1. Actinoid Polyps, illustrated in Figs. 137, 138, which make all 

 ordinary corals. The rays or tentacles 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 the spaces between fleshy 

 partitions in the interior of the polyp. 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. Alctonoid Polyps, illustrated in Fig. 139, which make 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 

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



