3o6 HYDRA CHAP. 



just as in a civilised community the functions of baking and 

 butchering are assigned to certain members of the com- 

 munity, and not performed by all. Hydra is therefore an 

 example of individuation : morphologically it is equivalent 

 to an indefinite number of unicellular organisms : but, 

 these acting in concert, some taking one duty and some 

 another, form, physiologically speaking, not a colony of 

 largely independent units (compare p. 277), but a single 

 multicellular individual. 



Hydra has two distinct methods of reproduction, asexual 

 and sexual. 



Asexual multiplication takes place by a process of budding. 

 A little knob appears on the body (Fig. 75 A, bd l \ and is 

 found by sections to arise from a group of ectoderm-cells ; 

 soon, however, it takes on the character of a hollow out- 

 pushing of the wall containing a prolongation of the enteron, 

 and made up of ectoderm, mesoglcea, and endoderm. (Fig. 

 76, A, bd l ). In the course of a few hours this prominence 

 enlarges greatly, and near its distal end six or eight hollow 

 buds appear arranged in a whorl (Figs. 75, A, and 76, A, 

 bcfi\ These enlarge and take on the characters of ten- 

 tacles, and a mouth is formed at the distal end of the bud, 

 which thus acquires the character of a small Hydra (Fig. 75, 

 A, bcP). Finally the bud becomes constricted at its base, 

 separates from the parent, and begins an independent ex- 

 istence. Sometimes, however, several buds are produced at 

 one time, and each of these buds again before becoming 

 detached : in this way temporary colonies are formed. But 

 the buds always separate sooner or later, although they 

 frequently begin to feed while still attached. 



It is a curious circumstance that Hydra can also be mul- 

 tiplied by artificial division : the experiment has been tried 



