46 James D. Dana, Esq., on the Structure, 



racter. The internal cavity is a simple tube without radiating 

 lamellae or special organs of reproduction, and the gemmules 

 grow out singly or in bunches from the sides of the 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 Hydrse. The 

 Hydroidea are all minute, and act no important part in reef- 

 making. 



The Hydroidea were long considered mature animals. But 

 recent investigations have shewn 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 Ascidise rather 

 than to zoophytes. In habit and size, they much resemble 

 the Hydroidea. From a minute cabin-like cell, they extend 

 a circlet of slender arms or tentacles, and expand into a de- 

 licate 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 intestine curves around and termi- 

 nates in the disk. They are widely removed from true zoo- 

 phytes, both by this character and also by having the ten- 

 tacles furnished with vibratile cilia, — that is, minute appen- 

 dages resembling short hairs, which are kept in nearly con- 

 stant vibration. Some species of Bryozoa form thin crusts 

 over rocks or sea-weeds, consisting of united cells, scarcely 

 distinguishable unless magnified. The corolla 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 



