CHAPTER II 



THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS 



13. Colonial Protozoa, When one of the simplest animals 

 multiplies by fission, the halves of the one-celled body sepa- 

 rate wholly from each other, move apart, and pursue their 

 lives independently. The original parent cell divides to 

 form two cells, which exist thereafter wholly apart from 

 each other. There are, however, certain simple animals 

 which are classed with the Protozoa, which show an inter- 

 esting and important difference from the great majority of 

 the simplest animals. These are the so-called colony-form- 

 ing or colonial Protozoa. 



These colonial Protozoa belong to a group of organisms 

 called the * Volvocinae. The simplest of the Volvocinae are 

 single cells, which live wholly independently and are in 

 structure and habit essentially like the other Protozoa we 

 have studied. They have, however, imbedded in the one- 

 celled body a bit of chlorophyll, the green substance which 

 gives the color to green plants and is so important in their 

 physiology. In this respect they differ from the other 

 Protozoa. Among the other Volvocinae, however, a few or 

 many cells live together, forming a small colony that is, 



* These colonial organisms, the Volvocinae, are the objects of some 

 contention between botanists and zoologists. The botanists call them 

 plants because they possess a cellulose membrane and green chroma- 

 tophores, and exhibit the metabolism characteristic of most plants ; but 

 most zoologists consider them to be animals belonging to the order 

 Flagellata of the Protozoa. In the latest authoritative text-book of 

 zoology, that of Parker and Haswell (1897), they are so classed. 

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