86 IXVEETEBRATE ANIMALS. 



will, whilst 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 disc is in turn surrounded by numerous 

 simple tubular tentacles, arranged in alternating rows. The ten- 

 tacles consist of both ectoderm and endoderm, enclosing a tube 

 which communicates with the body-cavity. By the muscular con- 

 traction of the walls of the column, the fluid contained in the 

 body-chamber can be forced into the tentacles, which can be thus 

 protruded to a great length, whilst they can also be usually re- 

 tracted. In some cases the tentacles are furnished with perforations 

 at their extremities. The mouth (fig. 47) 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 

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

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

 compartments by radiating vertical plates, which are called the 

 "mesenteries," and to the faces of which the reproductive organs 

 are attached, in the form of reddish bSnds, containing either ova or 

 sperm-cells. Below the stomach, the free edges of the mesenteries 

 are thickened, and constitute twisted threads or cords, which are 

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

 and in some related forms the base is tapering, and it appears that 

 the animal either lives freely in the sea, or buries itself to the lips 

 in the sand. 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, con- 

 sisting of numerous polypites united by a creeping fleshy trunk or 

 co'Kosarc. 



The second group of the Zoantharia is termed that of the Zoan- 

 iJhiria sclerodermata, from the nature of the skeleton or coral. In 

 cliis group are all the so-called " reef -building " Corals, which are the 



ijirincipal makers of the well-known "coral-reefs." The members of 

 this group all possess the power of secreting carbonate of lime within 

 their tissues, so as to form a more or less continuous skeleton or 

 , -orallum. From the fact that this corallum is secreted by the inner 

 / layer of the polypes, and is therefore trxAYjwithin the body, it is said 

 Ito be " sclerodermic," in opposition to the kind of coral produced by 

 other forms (such as the Red Coral), in which the coral for ms an 

 internal _axi3^_e2££. w'hidx -th&-XDeiLQSarc_is_spread, much as the bark 

 encloses the wood of a tree. In this latter case the coral is said to 



