STRUCTURE AND DEVELOPMENT 51 



by which it attaches itself, while at the up|)er end, 

 surrounded by a group of tentacles, is the mouth which 

 leads to the central cavity. The wall of this simple 

 body is composed of two layers of cells, between which 

 there is a gelatinous layer rarely invaded by cells. The 

 inner layer lines the central si)ace into which food 

 organisms are thrust by the tentacles, and it is con- 

 cerned primarily with digestion. The outer layer 

 comprises cells for protection and sensation prima- 

 rily. Cells of both layers have muscular prolongations 

 which by their operation enable the whole animal to 

 change its form and to move from one place to another. 



It may seem that such an animal is totally unlike 

 any of the higher and more complex types. In certain 

 respects, however, it is identical with the other forms 

 inasmuch as it performs all of the eight biological tasks 

 demanded by nature. It is also similar in so far as 

 its inner layer, like the innermost sheet of cells in 

 higher forms, is concerned with problems of taking 

 and preparing food, while the protective outer layer re- 

 sembles in function the outermost covering of all animals 

 higher in the scale. Beyond these a still more funda- 

 mental agreement is found in its cellular composition. 



At the lower end of the animal scale are organisms 

 which consist of one cell and nothing more. Arriaba, 

 to which we must refer again and again, is an example 

 of this group which possesses an overwhelming inij)or- 

 tance to the comparative student because the origins 

 of all the characteristics of animals higher in the scale 

 are to be found within it. Amoeba itself is a naked 

 mass of protoplasm, about -^J^ of an inch in diameter, 

 enclosing a nucleus. Its form is not constant during 



