CORALS. 



113 



animal swims in the water, and the sense of touch is necessary to supplement that of 

 sight in order to know where the animal is. In stony corals on the other hand we find 

 secreted in the animal tissues a very hard, cal- 

 careous substance which forms a skeleton, and, 

 when the secretions of a number of individuals 

 are united, an axis or head upon which the corals 

 build or secrete their skeletons. When we view 

 a white branch of coral we see nothing of the 

 animal save its dried and bleached skeleton, 

 from which most of the organic nature has dis- 

 appeared. If we could go to the tropics and 

 study the medusae and corals alive many points 

 of likeness might be traced in their general 

 structure. 



"We are accustomed to associate with all 

 animals a certain amount of motion from place 

 to place in the medium in which they live. In 

 the Actinozoa, however, we find very few of the 

 adults endowed with locomotive powers, and 

 all, as a general thing, are fixed to some foreign 

 body. The solid carbonate of lime which the 

 majority of the corals secrete in their tissues is, 

 in respect to its method of formation, identical 

 with all secretions in animal bodies. It is not 

 the work of the coral any more than the shell 

 of the clam or the covering of the lobster is the 

 work of the animals which they enclose and 

 protect. It is an internal or external formation 

 by secretion, and as a consequence coral animals 

 are not mechanical builders any more than any 

 other animal can be called the mechanical con- 

 structor of its shell or skeleton. Many of the 

 Actinozoa have no power of secreting hard 

 matter in the form of a skeleton, and the bodies 

 of such have a soft, gelatinous character, while 

 in their tissues, as in those of the medusae, a large proportion of water is found. The 

 solid secretions of coral animals, commonly known as coral, is sometimes erroneously 

 supposed to harden on exposure to the air. This opinion is probably well founded as 

 far as coral rock, a chemical product of the cementation of coral sand in a way to be 

 explained, is concerned, but is wrong as regards the coral in the foi-m secreted by the 

 animal. Althoi^h it appears to harden by the loss of animal matter, the skeleton 

 itself changes biit very slightly, if at all, in its hardness after the death of the animal 

 from which it came. 



The genus Actinia, a soft-bodied Actinozoan which has not the power of secreting 

 -carbonate of lime, is for various reasons the one commonly taken to illustrate the 

 anatomy and characteristics of the group. Actinia, found in almost all seas in the 

 temperate as well as torrid zones, is represented by a large number of species, the ma- 

 jority of which are of comparatively large size. The Sea-Anemone, Metridium 



VOL. I. — 8 



Fig. 106. — Mmioxenia, young halcyonoid polyp. 



