90 INVERTEBRATE ANIMALS. 



thirty-three thousand square miles. Its average distance from the shore is 

 between twenty and thirty miles, the depth of the inner channel is from ten 

 to sixty fathoms, and the sea outside is "profoundly deep " (in some places 

 over eighteen hundred feet). 



3. Atolls (Fig. 2Y, 3). — These are oval or circular reefs of coral enclosing 

 a central expanse of water or lagoon. They seldom form complete rings, 

 the reef being usually breached by one or more openings. They agree in 

 all particulars with those barrier-reefs which surround islands, except that 

 there is no central island in the lagoon which they enclose. 



The last group of the Zoantharia comprises composite or- 

 ganisms in which the coenosarc is supported upon a central 

 axis or sclerobasic skeleton. These Zoantharia sclerobasica 

 require no notice, except simply to remark that they are dis- 

 tinguished from other sclerobasic corals (such as the Gor- 

 gonidoe) by the fact that each polype possesses tentacles which 

 are a multiple of six in number. 



Oedee II. Alctonaeia. — The second great order of living 

 Actinozoa is distinguished by the fact that the polypes are 

 furnished with fringed tentacles, and that these, as well as 

 the mesenteries and somatic chambers, are always some mul- 

 tiple of four. With one doubtful exception, all the Al- 

 cyonaria are composite, their polypes being connected to- 

 gether by a ccenosarc. The body-cavities of the polypes are 

 connected with a system of canals which are excavated in the 

 coenosarc, and communicate freely with one another, so that a 

 free circulation of nutrient iluids is thus kept up. The struct- 

 ure of the polypes of the Alcyonaria is, in all essential 

 anatomical features, the same as in the sea-anemones, the 

 number of the mesenteries and tentacles being the chief dis- 

 tinction. 



Of the various different organisms included under this order, 

 one of the best known is the " Dead-men's-fingers," or Alcyo- 

 nium, which occurs commonly in most seas. It forms spongy- 

 looking masses of a yellow or orange color, attached to shells 

 and other marine objects. The whole mass is covered with 

 little star-shaped apertures, through which the delicate pol- 

 ypes can be protruded and retracted at will. Another well- 

 known member of this order — the type of another family — is 

 the " sea-rod " ( Virgularia mirabilis), which occurs not very 

 rarely in shallow seas. Virgularia occurs in the form of a long 

 rod-shaped body of a light flesh-color, supported upon a cal- 

 careous rod, somewhat like a knitting-needle, which is covered 

 by the coenosarc. From the coenosarc are given out lateral 



