112 POPULAR ILLUSTRATIONS OF 



Now, by the vital force inherent in the polyp, a power over which 

 the creature has no control, there is formed by the outer membrane, 

 or ectoderm, a calcareous mass, which I have marked a. 



We will suppose that the polyp has just arrived at polyphood, 

 if I may coin a word, and is about to become the founder of a 

 great colony of polyps. Well, then, having formed a starting point 

 at , the polyp increases itself by buds, or self -division ; and the 

 progeny, connected with each other and their respected parent by 

 an extension of the ectoderm constituting a coenosarc, also secrete 

 Coral. 



The form, the fashion, the direction of this secreted Coral are 

 determined by the individual necessity of the polyp family. In one 

 case it rises up a column like that of Fig. 148, which divides like a 

 tree, the stem and branches being equally covered by the compound 

 Actinozoon. When taken from the bottom of the sea and the " live 

 flesh " cleared off, this turns out to be the red Coral of commerce, of 

 which our shirt-studs- and ladies' ornaments are made. 



Another species prefers horn to lime, and forms the beautiful 

 skeleton known as the Sea-Fan (see frontispiece) ; while still another 

 species, having, I suppose, a literary taste, shapes its skeleton in the 

 form of a pen, which shines at the bottom of the sea with a phos- 

 phorescence brilliant and beautiful to behold (Fig. 164, frontispiece). 



Now there are three facts to be noted by the student in reference 

 to the Alcyonaria polyps. 



First, they have eight tentacles or multiples thereof. Secondly, 

 that, although the Coral appears to be within the living mass of 

 polyps, being secreted by the outer skin, it is in reality outside the 

 animal ; and, thirdly, being formed by the basal part of the animal, 

 and the Coral being very hard, naturalists have called the mode of 

 its production by the name of sclero-basic (from skleros, hard ; and 

 "base"). 



If, however, the student prefers it, he may remember this, accord- 

 ing to Dana, as Coral formed lay foot secretion ; not that the creature 

 has a foot, but simply that the part of its body which secretes the 

 Coral performs the office of a "foot." 



Now, if you please, just transfer your attention to Fig. 149, 

 which is another diagrammatic section of a Cqral polyp, and of by 

 far the most numerous family, among which are the Coral reef 

 builders. 



Observe that the skeleton is formed in this section by the inner 

 skin, or endoderm, and that it extends from without inwards, follow- 

 ing, in fact, the course of the partitions or mesenteries which I illus- 

 trated in the sea-anemones in the last chapter, and which may be 

 further seen in the transverse section of a corallurn (Fig. 151, p. 115). 



