86 INVERTEBliATE ANIMALS. 



will, whilst the mnut'.i is jilaced 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 endoderni, enclosing a tube 

 which connnunicates 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 caies the tentacles aie furnished with perforations 

 at their extremities. The mouth (tig. 47) leads directly into the 

 stomach, which is a wide membranous tul_ie, opening by a wide 

 aperture into the bodj'-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 inunber of separate 

 compartments Ijy radiating vertical jilates, which are called the 

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

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

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

 are thickened, and constitute twisted threads or coids, 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 exjianded 

 base. They are not, however, permanently fixed, but can cha)ige 

 their place at will. In the nearly allied Ili/anthus and Arac/iiuietis 

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

 tlie animal either lives freely in the sea, or buries itself to the li|is 

 in the saml. The true Sea-anemones, as already said, are all simple, 

 each consisting of a single ])nlype; but there are closely related 

 forms (such as Zmiiit/iiix') m which the organism is compound,' con- 

 sisting of numerous jjolj'pites united by a creeping fleshy trunk or 

 ccenosarc. 



The second grou]) of the ZmintJiarin is termed that of the Zonn- 

 tharia sdcrodennata , from the nature of the skeleton or coral. In 

 this group are all the so-called "reef-binlding " Corals, which are the 

 ])i-inci])al makers of the well-known "coral-reefs." The members of 

 this groviji all possess the jMJWer of seci'eting carbonate of lime within 

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

 coralluni. From the fact that this coralluni is secreted by the inner 

 layer of the polypes, and is tlici-efore truly iritliin the body, it is said 

 to be " .sclenideiniic," in o]>piisitiiiii to the kind of coral produced by 

 other foruis (such as the Keil Coral), in which the coral forms an 

 .iiteTiial axis, over which the crenosarc is s]irea,d, much as the bark 

 e};clc)ses the wood of a tree. In this latter case the na-.al is said to 



