230 HYDRA LESS. 



The essential difference between digestion and assimilation 

 is here plainly seen : all the cells of Hydra assimilate, all 

 are constantly undergoing waste, and all must therefore form 

 new protoplasm to make good the loss. But it is the endo- 

 derm cells alone which can make use of raw or undigested 

 food : the ectoderm has to depend upon various products of 

 digestion received by osmosis from the endoderm. 



It will be evident from the preceding description that 

 Hydra is comparable to a colony of Amoebae in which par- 

 ticular functions are made over to particular individuals 

 just as in a civilized community the functions of baking and 

 butchering are assigned to certain members of the commu- 

 nity, and not performed by all. Hydra is therefore an ex- 

 ample 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, but a single multicellular in- 

 dividual. 



Like many of the organisms which have come under 

 our notice, 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. 49, A, b<P-\ 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, mesogloea, and endoderm. (Fig. 

 50, 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 (Fig. 49, A, bd? ; Fig. 50, 



