RADIATA. 173 



plete. Hence the wonderful powers of propagation in 

 these creatures, which have astonished naturalists ever 

 since Trembley first discovered them, in 1744. He says : 

 " I have opened a polyp on my hand, extended it, and 

 cut the simple skin of which it is formed in every direc- 

 tion ; I have reduced it to little pieces, and, in a man- 

 ner, minced it. These little pieces of skin, both those 

 which did and those which did not possess arms, became 

 perfect polyps." Many curious multiple forms have 

 been ' produced by experiments on these animals. By 

 slitting the body into two branches, and these branches 

 again into others, a tree-like form may be produced, each 

 branch giving rise to a distinct head and tentacles. Or 

 one may be turned inside out like a glove, so that the 

 outer skin becomes the lining of the stomach-cavity, with 

 a transposition of the functions of each. 



Order I. Hydroida. This order is composed of ani- 

 mals built on the pattern of the Hydra, just described. 

 They are either single, as the Hydra, or compound. The 

 latter are subdivided into the three families of Campanu- 

 larians, Sertularians, and Tubularians. They are grouped 

 in clusters or colonies on a common axis or stalk, (cceno- 

 sarc^) Each hydra-like organism is called a polypite. 

 New polypites arise as outgrowths from the common 

 stem of the colony, so that the stomach of each is con- 

 tinuous with the tubular center of the stalk, producing a 

 community of nutrition in the colony. In Chap. III., 

 Sec. 14, reference was made to the alternation of genera- 

 tions which this order of animals so strikingly illustrates. 

 This process in the life-history of the Hydroids is briefly 



as follows : The Polyp, a fixed animal, increases for 

 15* 



