GEIATLXOSI. 655 



their reach. Light affects them very powerfuuy, and they are fond of it. By division of the hody 

 they may be multiplied to an indefinite extent ; but their natural production is by buds, which shoot 

 out from various parts of the parent animal, and drop off when they are matured. They are found in 

 stagnant waters, usually under the floating leaves of aquatic plants ; and it is understood that they tend 

 to purify the waters. Some are green, others of a grey colour, and they vary also in size. 



Corine, have a fixed stem and oval body, open at the summit, and covered with little tentacula. Their texture 

 is firmer than that of Hydra ; some of them carry the ova on the under part of the body, in a manner similar to 

 that of some Crustacea and Arachnidse. 



Cristatella, have over the mouth a double range of numerous tentacula, forming a sort of plume in the shape of 

 a half-moon, the regular motion of which brings food to the animal. These mouths are on short necks attached 

 to a gelatinous body, which moves somewhat similar to Hydra. They inhabit stagnant waters ; but to the naked 

 eye, they appear only as little spots of mould. 



Vorticella, have the stem fixed, often much branched and divided, with a bell or horn-shaped termination to 

 each branch, and two opposite groups of filaments, which agitate the water. They abound in stagnant fresh 

 waters, and are arranged as bushes, shrubs, plumes, and other agreeable forms ; but they are too minute for being 

 seen by the naked eye. 



Pedicellaria, are found between the spines of Echini, and by some considered as organs of these animals, but the 

 probability is that they are Polypi, which seek shelter there. They consist of a slender stem, with a horn on the 

 tip, furnished with tentacula like minute threads or leaves. 



THE THIRD ORDER OF THE POLYPI. 



CORALLIFERI. 



These include all those numerous species, which were for a long time regarded as marine 

 plants, and in which numerous individuals are so united as to form compound animals, for the 

 most part fixed like plants by a branched stem, or by simple expansions of a solid substance, 

 at the base, or in the middle of the group. The individual animals, which are more or less 

 analogous to Actinia and Hydra, are all connected in a common body, and have a general 

 nutrition, so that whatever one eats, tends to the nourishment of the common body, and of all 

 the individuals. Their instincts appear also to be common, at least in those species which 

 have free motion in the water, for they swim by the joint action of the general body, and of 

 all the Polypi. Polypidom (the House of the Polypi), is the name usually given to the common 

 part of these compound animals ; but the name is not quite correct, inasmuch as the common 

 part is sometimes internal, and sometimes external. These polypidoms are formed in layers 

 by deposition, somewhat similar to the ivory of teeth ; and they are of various degrees of 

 hardness ; the hind parts being composed of salts of lime, but always united by means of 

 animal matter, in the same manner as the lime in bones, crusts, and shells. The differences 

 of form and situation in the polypidoms, gives rise to many divisions and subdivisions. 



THE FIRST FAMILY OF THE CORALLIFERI. 

 The Tubularia. 



These inhabit tubes which have a common gelatinous stem pervading the axis, like the pith of a tree ; 

 and the tubes open sometimes on the summit, and sometimes at the sides, for allowing a passage to the 

 Polypi. These Polypi arc individually very simple, and resemble in their organization Hydra and Cris- 

 tatella. 



They form three principal genera, but each admits of subdivision. 



Tubipora, — 

 Have the tubes simple, and of stony consistence, each containing a simple To-hpc, and arranged parallel 

 like the pipes of an organ. 



T. musica, abundant in the Oriental Archipelago, has the tubes of a fine red, and tlu> polypi green and like Hydra. 

 Some fossil polypidoms, such as Cateiiipora, in which the tubes are disposed in ""ftMtj ami Ftmwitm, where they 

 are crowded and hexagonal, resemble this genus. 



