26 COMPARATIVE PHYSIOLOaY. 



A Polyp is like a colony of Amoebae in which some division of 

 labor (function) has taken place ; a sort of biological state in 

 which every individual is nearly equal to his neighbor, but 

 somewhat more advanced than those neighbors not members of 

 the organization. 



But in one respect the Polyps show an enormous advance. 

 Ordinarily when nourishment is abundant Hydra multiplies by 

 budding, and when cut into portions each may become a com- 

 plete individual. However, unde^r other circumstances, near 

 the bases of the tentacles the body wall may protrude into little 

 masses (tes es), in which cells of peculiar formation (sperma- 

 tozoa) arise, and are eventually set free and unite with a cell- 

 {ovum) formed in a similar protrusion of larger size (ovary). 

 Here, then, is the first instance in which distinctly sexual repro- 

 duction has been met in our studies of the lower forms of life. 

 This is substantially the same process in Hydra as in mammals. 

 But, as both male and female cells are produced by the same 

 individual, the sexes are united (hermaphroditism) ; each is at 

 once male and female. 



Any one watching the movements of a Polyp, and compar- 

 ing it with those of a BeU-animalcule, will observe that the 

 former are much less machine-like ; have greater range ; seem 

 to be the result of a more deliberate choice ; are better adapted 

 to the environment, and calculated to achieve higher ends. In 

 the absence of a nervous system it is not easy to explain how 

 one part moves in harmony with another, except by that pro- 

 cess which seems to be of such wide application in nature, adap- 

 tation from habitual simultaneous effects on a protoplasm capa- 

 ble of responding to stimuli. When one process of an Amoeba 

 is touched, it is likely to withdraw all. , This we take to be due 

 to influences radiating through molecular movement to other 

 parts ; the same principle of action may be extended to Hydra. 

 The oftener any molecular movement is repeated, the more it 

 tends to become organized into regularity, to become fixed in 

 its mode of action ; and if we are not mistaken this is a funda- 

 mental law throughout the entire world of living things, if not 

 of all things animate and inanimate alike. To this law we 

 shall return. 



But Hydra is a creature of but very limited specializations; 

 there are neither organs of circulation, respiration, nor excretion, 

 if we exclude the doubtful case of the thread-cells (urticating 

 capsules). The animal breathes by the entire surface of the 



