32 



POLYPIFERA. 



free, and change their position by means of 

 their vibratile cilia. When the polypes are 

 expanded, the sea-water has a free passage 

 through the stomach to the gemmules, which 

 receive new vigour from its influence, and 

 they advance towards the open posterior part 

 of the stomach, become entangled in its aper- 

 ture, pass through the stomach, and escape 

 through the mouth. They then move about, 

 by means of their cilia, in search of a place 

 where to fix and develope themselves. 



The stony branches of the Coral are suffi- 

 ciently short and strong to resist the violence 



of the sea, which otherwise would break so 

 fragile a substance, but in the Gorgoniae and 

 Antipathes the ramifications are so long and 

 slender, that they would inevitably be broken 

 by the movements of the surrounding water, 

 were it not that the nature of their central 

 axis is materially modified. Tins part of their 

 structure is therefore very considerably modified 

 in its texture, and being composed of flexible 

 materials is enabled to bend beneath the passing 

 current and rise again uninjured, while in the 

 Isis Hippnris (Jig. 37) a similar result is ob- 

 tained by combining the horny and calcareous 



Fig. 37. 



Isis Hippurls. 



matter in alternate joints. In these latter 

 polyparies, however, although their central 

 axes are principally composed of corneous 

 substance disposed in concentric layers, the 

 living cortex itself is full of granules of a cal- 

 careous nature mixed with colouring matter 

 that varies in different genera, and as this 

 cortex dries in a thick layer upon the central 

 stem when the Gorgonia is removed out of 

 the water, the varieties of colour exhibited 

 bv these zoophytes is conspicuous even in 

 our cabinets. Cavolini* prosecuted for two 

 successive years, 1784- and 1785, his researches 

 on the structure of Gorgonia vcrrucosa (Lam.), 

 * Abhandlung ueber Pflauzen-tbiere, p. 48. 



and found the anatomy of the polypes dis- 

 persed over the surface of each branch to be 

 similar to that described above as common to 

 the Alcyonidae and Coral ; he detected the 

 position of the ovaria at the base of each 

 polype, and observed that the ova were dis- 

 charged through eight small oviducts that 

 open between the bases of the eight tenta- 

 cula. These ova he describes as ciliated 

 gemmules which, on their escape, swim to 

 and fro in the surrounding water, and asserts 

 that he saw a portion of Gorgonia, only eight 

 inches high, discharge ninety of these in the 

 space of an hour from the different polypes 

 studding its surface. 



