INTEODTJCTION. 17 



indications of a vegetable nature. They are not only fixed 

 to the ground like plants, but they have also a plant-like 

 method of growing and propagating. 



It is extremely difficult to convey any general idea of a 

 zoophyte, because there is no order of creatures of which 

 the different individuals bear so little resemblance to each 

 other. The organization of the corallines, flustras, sertu- 

 larias, and other orders of marine zoophytes may, however, 

 be illustrated by that of the Hydra, or common fresh-water 

 polype. These animals, which resemble little pieces of jelly, 

 are found in ponds or slowly running streams, attached to 

 the under surface of the leaves of aquatic plants, or to any 

 floating substance, such as a stick or a straw. They are 

 remarkable for the extreme simplicity of their organiza- 

 tion, which consists of nothing but a digestive cavity or 

 stomach, surrounded by a fringe of long thread-like arms 

 or movable tentacula, by means of which the animal pro- 

 cures its food — generally some minute insect or worm, 

 which it seizes with a tendril-like grasp and instantly con- 

 veys into its stomach by a contractile effort. The contrac- 

 tility of the tentacula of the Hydra is truly wonderful. 

 When the animal is hungry and in search of prey, the ten- 

 tacula are extended to a distance of not less than six or 

 seven feet from the mouth of the stomach ; but when the 

 digestive cavity is filled with food and the wants of the 

 animal are appeased, they are so contracted as to appear 

 only like tubercles around its entrance. 



The different species of corallines, flustras, and sertu- 

 larias, usually found attached to, or more frequently 

 intermingled with, the sea-weed cast upon our shores, con- 

 sist of an association of polypes having individually a simi- 

 lar organization to the Hydra, but united together about 

 a common axis of growth like the buds and branches of 



2* 



