490 CHARACTERS OF INVERTEBRATE ANIMALS 



its shape, blunt lobes of the protoplasm (pseudopods) being pro- 

 truded from time to time. They are not constant structures, 

 but can be formed and again obliterated at any part of the body — 

 at least in the commonest species of Amoeba. Not only do these 

 lobes enable the animal to crawl about, but they also serve to 

 secure food particles, engulfing them, so to speak. No breach 

 is formed thereby, for any hole temporarily made in the semi- 

 fluid protoplasm at once closes up without leaving any trace. 

 Once within the protoplasm the food is digested, and the un- 

 digested portions of it are cast out from any part of the body. 

 In the case of Hydra some of the endoderm cells take in and 

 digest solid particles in much the same way (p. 471). 



The simple body of Amceba presents but little distinction of 

 parts, but the outer part of the protoplasm is clearer and perhaps, 

 somewhat denser than the inner part into which the food is taken, 

 and which also contains, as a rule, numerous granules of various, 

 nature. As in a colourless blood corpuscle a small rounded 

 particle of specialized protoplasm, the nucleus, can be distinguished, 

 and this appears to have a great deal to do with regulating and 

 controlling the nutrition and other functions. Its presence shows 

 that AmcEba is a single cell or unit of structure, i.e. is unicellular. 

 The body of Hydra, and the same thing is true for all animals 

 higher than the Protozoa, has been compared (p. 469) to a house 

 composed of various building materials of which the units are 

 bricks, blocks of stone, and the like, these materials being likened 

 to the tissues of the animal in which cells of differing kind are 

 the constituent units. An unicellular creature like Amoeba may 

 therefore be compared to a house built of one brick, if such a 

 metaphor can be regarded as thinkable. The Protozoa indeed 

 are defined as animals in which the body is made up of one cell 

 only, or at most of an aggregate of cells which are not specialized 

 into tissues, so that each member of the aggregate has to perform 

 all the functions of life. The remaining thirteen phyla of animals 

 are often grouped together as Metazoa, in which the body is 

 made up of more or less numerous cells specialized to form tissues. 

 It may be as well to express this in tabular form, advantage also 

 being taken of the opportunity to show the limits of Vertebrata 

 and Invertebrata, of Diploblastica and Triploblastica. 



There can be no doubt that the Metazoa are descended from 

 unicellular forms, which must have resembled some of the 



