74 



SYSTEMATIC ZOOLOGY 



and the position of the muscles on only one side of the mesen- 

 teries make it possible to pass a plane through the body so as 

 to divide it into two equal parts, thus producing a bilateral 

 symmetry (Fig. 56). In these colonies the individuals are all 



united by a coenosarc as in the 

 hydroid colonies. Some Anthozoa 

 have no skeleton, but the majority 

 produce a supporting structure, 

 which consists of calcium carbonate 

 or in some cases of a substance 

 resembling but not identical with 

 horn; at times both kinds occur in 

 one species. The skeleton is usu- 

 ally a compact, solid structure, but 

 in some cases it consists of loosely 

 arranged spicules. It is usually a 

 secretion of the ectoderm, though 

 in some cases it may be formed by 

 Mf the mesenchyme cells, and both 

 kinds of cells take part in secreting 

 the skeleton in some species. Some 

 of the polyps of this class live sir. _ 

 but by far the greater number form 

 colonies, by budding, often very large 

 and of various shapes, such as cup- 

 shaped, dome-shaped, and branching 

 ( Figs. 59 and 60). Reproduction 



Fig. 58. Longitudinal section through 



an anthozoan polyp. G, germ-glands ; also takes place by means of fer- 



M, oesophagus; Mf mesenteric folds, tilized eggS, which give rise to 

 Enlarged. (After Claus.) && 



ciliated, tree-swimming larvae. The 

 larva eventually settles down and develops into a polvp, which 

 lives by itself or produces a colony by budding, as the case 

 may be. 



Many of the Anthozoa are very brilliantly colored. The great 

 variety exhibited by the sea anemones is familiar to all who live 

 by the sea shore ; but the living corals are even more beautiful, 

 and are less known to the inhabitants of the temperate zone, 

 for they live for the most part in tropical waters, and one gains 

 no conception of the living animal by seeing the skeleton only. 



