20 CORALS AND COMAL ISLANDS. 



present time, in a former age of the world — the Paleozoic — 

 they so abounded over the sea bottom that some beds of lime- 

 stone are half composed of them. 



Fourth, Algse or sea-weeds, some kinds of which would 

 hardly be distinguished from corals, except that they have no 

 cells or pores. 



I. POLYPS. 



A good idea of a polyp may be had from comparison with 

 the garden aster ; for the likeness to many of them in external 

 form as well as delicacy of coloring is singularly close. The 

 aster consists of a tinted disk bordered with one or more series 

 of petals. And, in exact analogy, the polyp flower, in its 

 most common form, has a disk fringed around with petal-like 

 organs called tentacles. Below the disk, in contrast with the 

 slender pedicel in the ordinary plant, there is a stout cylindri- 

 cal pedicel or body, often as broad as the disk itself, and some- 

 times not much longer, which contains the stomach and inter- 

 nal cavity of the polyp ; and the mouth, which opens into the 

 stomach, is at the centre of the disk. Here then the flower- 

 animal and the garden-flower diverge in character, the dif- 

 ference being required by the different modes of nutrition and 

 other characteristics in the two kingdoms of nature. The cor- 

 al polyp is as much an animal as a cat or a dog. 



The figures of the frontispiece, and others on pages 23, 24, 

 26, sustain well the description here given, and afford some 

 idea also of the diversity of form among them. 



The prominent subdivisions of polyps here recognized are 

 the following: 



I. Actinoid Polyps. — Belated to the Actinia, or Sea-anem- 

 one, in tentacles and interior structure, and having, as in 



