g2 INVERTEBRATE ANIMALS. 
swimming organisms. Some of them are unprovided with hard 
structures or supports of any kind, as the sea-anemones and 
Ctenophora; but a great many 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 Rugosa, and the Ctenophora. 
ORDER I. ZOANTHARIA.— The Zoantharia comprise those 
Actinozoa in which the polypes are furnished with ssoo¢h, 
stniple, usually numerous tentacles, which, like the mesenteries, 
_ are in multiples of #ve or six. The Zoantharia are divided 
into three groups, distinguished from one another by the pre- 
sence 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 
c 
Se 
é€e: 
aa ee 
-— ee 
Fig. 33.—Morphology of 4 ctinide. a Actinia rosea; b Arachnactis albida. 
(After Gosse.) 
merely of little scattered needles or spicules of carbonate ot 
lime. Generally, too, the organism is simple, and consists of 
no more than a single polype. The best known of the mem- 
bers of this group are the beautiful sea-anemones or animal- 
flowers (Aciznide), which occur so plentifully on every coast 
(fig. 33, 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 
