TEXTURE AND COMPOSITION OF CORALS. 55 



animal. The Hydra, an animal a line or two in length, consists 

 of a tubular body, with a mouth at one extremity surrounded by 

 a circle of tentacles : and the structure of the animal is so simple 

 that it may be turned inside out, and still live and eat; it may 

 be cut into forty or more parts, and from the dissected body, will 

 grow as many distinct Hydras. The Hydroidea are all minute 

 and act no important part in reef-making. 



The Hydroidea were long considered mature animals. But re- 

 cent investigations have shown that part at least develop Medusae, 

 which are properly the adult individuals since these alone produce 

 true ova. This division has therefore been recently removed 

 from the zoophytes and placed with the Acalephs or Jelly-fishes. 



The Bryozoa. — The Bryozoa are other coral-making species ; 

 but they are related to certain molluscs called Ascidiae rather than 

 to zoophytes. In habit and size they much resemble the Hy- 

 droidea. From a minute cabin-like cell, they extend a circlet of 

 slender arms or tentacles, and expand into a delicate goblet-shape 

 flower, seldom over a line in diameter. These polyps differ both 

 from the Actinoidea and Hydroidea, in having two extremities 

 to the alimentary canal — an anus, as well as a mouth ; the intes- 

 tine curves around and terminates in the disk. They are widely 

 removed from true zoophytes, both by this character, and also 

 by having the tentacles furnished with vibratile cilia — that is, 

 minute appendages resembling short hairs, which are kept in 

 nearly constant vibration. 



Some species of Bryozoa form thin crusts over rocks or sea- 

 weeds, consisting of united cells, scarcely distinguishable unless 

 magnified. The coralla of other species are branching or thin 

 foliaceous ; and these also consist of series of minute cells. 



2. Texture and Composition of Corals. 



The texture of calcareous corals is in general quite porous or 

 cellular. Small stars or rounded depressions are scattered over 

 the surface, and sometimes these stars form the centres of small 

 prominences, called calicles (little cups). Besides these polyp- 

 cells, which mark the position each of a separate polyp, there are 

 pores or cellules penetrating the texture of the coral mass ; yet 

 in some zoophytes, the coral secretions continue increasing in 

 the animal till the pores are almost or quite obliterated, and the 

 texture is nearly compact, the polyp cells alone remaining. In 

 many species, wherever there are concavities of much depth in 

 the surface of a zoophyte, the coral of these concavities is looser 

 or more spongy than elsewhere, for the reason, apparently, that 

 the polyps in such parts have a poorer chance for securing food 

 and fresh portions of water. 



In the Gorgoniae, and other species forming a distinct axis to 

 the branches, this axis is solid, without a trace of a cell, and 



