84 POPULAR HISTORY OF THE AQUARIUM. 



The most simple form of polype is that presented by the 

 Byclra, Clava, and others^ in which the body of the animal 

 is a simple sac, open at one end, and having the opening 

 surrounded by contractile threads or filaments, called tenta- 

 cles; while the other end has a sort of sucking disc, by 

 w^hich it adheres to other substances. The cavity of the 

 sac is the stomach ; the orifice of the sac represents a mouth ; 

 the tentacles surrounding it act as arms ; and the sucker at the 

 opposite end may be called a foot, since it secures adhesion 

 when at rest, and acts as an organ of locomotion when the 

 animal requires it. Many of the Zoophytes, including the 

 Actinia, a few Corals, and the Hydra before mentioned, live 

 singly, and throwing out their young through their mouths 

 leave them each to find an independent mode of existence ; 

 but some bud out at the sides and form branches, in such 

 a manner that each Zoophyte is a branched shrub, with a 

 common stem, composed of a large aggregate number of 

 individual polypes. 



Mr. Gosse divides the Polypiferous Zoophytes into two 

 Orders, thus : — 



Internal cavity simple, increasing^by germs growing out 

 from the sides, — " Hydroida." 



Internal cavity enclosing the stomach, and divided into 



