86 



INVERTEBRATE ANIMALS. 



secrete a calcareous or horny skeleton or framework which is 

 known as the " coral " or " corallum." 



The Actinozoa are divided into four orders^-viz., the Zoan- 

 tharia, the Alcyonaria, the Hugosa, and the Ctenophora. 



OEDER I. ZOANTHAEIA (Gr. zoon, animal 



anthos, flower). 



The Zoantharia comprise those Actinozoa in which the 

 polypes are furnished with smooth, simple, usually numerous 

 tentacles, which, like the mesenteries, are in multiples of five 

 or six. The Zoantharia are divided into three groups, dis- 

 tinguished from one another by the presence or absence of a 

 coral, and by its structure when present. 



The first of these groups is termed Zoantharia malacoder- 

 mata, or " soft-skinned " Zoantharia, because the polypes are 

 either wholly destitute of a coral, or, if there is one, it consists 

 merely of little scattered needles or spicules of carbonate of 

 lime. Generally, too, the organism is simple, and consists of 



FIG. 26. Morphology ofActinidce. a Actinia rosea ; & Arachnactis albida (after Gosse).. 



no more than a single polype. The best known of the mem- 

 bers of this group are the beautiful sea-anemones or animal- 

 flowers (Actinidce), which occur so plentifully on every coast 

 (Fig. 26, a). It will be as well to describe the structure of a 

 sea-anemone somewhat in detail, as in this way a clear notion 

 may be obtained of the general anatomy of the Actinozoa. 

 The body of an ordinary sea-anemone (Fig. 26, a) is a truncated 

 cone or short cylinder, termed the " column," and is of a soft, 

 leathery consistence. The two ends of the column are termed 



