CHARACTERISTICS OF POLYPI. 97 



polypi, similar to the parent; hence result masses of various 

 form, in which an entire series of generations are aggregated, 

 and seem to possess a life in common, just as if it were really a 

 compound creature, provided with a single body, possessing a 

 thousand mouths, and as many stomachs (fig. 88). In general 

 the digestive cavities of all these aggregated animals, living thus 

 in society, do not open directly into each other, but commonly 

 there are vascular communications between the individuals united 

 in a single mass, and the alimentary matter digested by one may 

 in this way be of advantage to all its neighbours. 



17. Frequently the bodies of these little animalcules is com- 

 posed entirely of a semi-transparent tissue of extreme delicacy ; 

 but in most of them the inferior portion of the tegumentary sheath 

 becomes much indurated, and even ossified so as to acquire the 

 hardness and appearance of stone. This solid envelop assumes 

 various forms, and sometimes constitutes tubes, and sometimes 

 merely cells ; for a long time it was considered merely as the 

 dwellings of the polyps which formed it, and is designated under 

 the name of coral. Sometimes every polyp has a distinct coral, 

 but ordinarily it is the portion common to an aggregated mass of 

 polyps that possesses the characteristics of these bodies, the 

 volume of which may become enormous, although each of the 

 parts forming it is extremely small. 



18. It is in this way that polyps of only a few inches in length 

 raise reefs and islands in seas bordering the tropics ; when placed 

 under circumstances favourable to their development, certain 

 animals of this class multiply to such a degree as to cover chains 

 of rocks or immense submarine banks, and form, with their stony 

 corals heaped one upon another, masses whose extent is con- 

 stantly increasing by the birth of new animalcules added to those 

 already existing. The solid slough or remnant of each colony 

 of polyps remains after the frail architects have perished, and 

 serves as a base for the development of other polyps, until these 

 living reefs reach the surface of the water, where these animals 

 cease to exist, and the soil formed by their remains ceases to 

 rise; but the surface of these masses of corals, exposed to the 

 action .of the atmosphere, becomes the site of a new series of 

 phenomena ; seeds, which are deposited by the winds, or borne 

 thither by the waves, germinate, and the surface of these coral 

 masses is in this way gradually clothed in a rich vegetation ; and 

 thus, what were but recently vast charnel-houses of almost micro- 

 sc [ ic zo'ophytes, are converted into habitable islands. In the 

 Pacific Ocean there are innumerable reefs and islands which had 

 no other origin; in general they seem to be based on the crater 



1 7. What is coral ? 



18. How are coral reefs formed ? 



9 



