ACTINOZOA. 87 



respectively the " base " and the " disk," the former constitut- 

 ing a kind of sucker, by means of which the animal can attach 

 itself at will, while the mouth is placed in the centre of the 

 latter. The mouth is surrounded by a flat space, destitute of 

 appendages, and the circumference of the disk is in turn sur- 

 rounded by numerous simple tubular tentacles, arranged in 

 alternating rows. The tentacles consist of both ectoderm and 

 endoderm, enclosing a tube which communicates with the 

 body-cavity. By the muscular contraction of the walls of the 

 column, the fluid contained in the body-chambers can be forced 

 into the tentacles, which can be thus protruded a great length, 

 while they can also be usually retracted. In some cases the 

 tentacles are furnished with perforations at their extremities. 

 The mouth (see Fig. 12, a) leads directly into the stomach, 

 which is a wide, membranous tube, opening by a wide aperture 

 into the body-cavity below, and extending about half-way be- 

 tween the mouth and the base. The wide space between the 

 stomach and body-walls is subdivided into a number of sepa- 

 rate compartments by radiating vertical plates, which are 

 called the " mesenteries," and to the faces of which the re- 

 productive organs are attached, in the form of reddish bands, 

 containing either ova or sperm- cells. Below the stomach, 

 attached to the free edges of the mesenteries, are a series of 

 singularly twisted threads or cords (Fig. 12, c), which are 

 filled with thread-cells, and are termed "craspeda." The 

 function of these is not well understood ; but it is believed 

 that in some cases they can be emitted through apertures, 

 which are occasionally found in the walls of the column. The 

 sea-anemones are mostly to be found between tide-marks, in 

 rock-pools, or on ledges of stone, adhering by means of the 

 expanded base. They are not, however, permanently fixed, 

 but can change their place at will. In the nearly allied 

 llyanthus and Arachnactis (Fig. 26, I) the base is tapering, 

 and it appears that the animal spends the greater part of its 

 existence in an unattached, free condition. The true sea- 

 anemones, as already said, are all simple, each consisting of 

 a single polype ; but there are closely-related forms (such as 

 Zoanthus) in which the organism is compound, consisting of 

 numerous polypes united by a creeping, fleshy trunk or cceno- 

 sarc. 



The second group of the Zoantharia is termed that of the 

 Zoantharia sclerodermata, from the nature of the skeleton or 

 coral. In this group are all the so-called "reef-building" 

 corals, which are the makers of the well-known " coral-reefs." 



