18 



INTRODUCTION TO ZOOLOGY 



CHAP. 



FIG. 8. A Colony of Car 

 chesium spectabile. 



only with its reproduction and dispersal. These latter forms 

 break away from the colony, and after a free -swimming 

 period settle down, and develop into 

 ordinary nutritive individuals, which 

 by fission give rise to new colonies. 

 Such a form, with two kinds of indi- 

 viduals in the colony, is seen in the 

 genus Zoothamnium common in sea- 

 water. 1 



In none of these colonies is the 

 union of the individuals of the colony 

 very vital, but we can readily imagine 

 the separate cells becoming more and 

 more closely united, and the division 

 of labour amongst them becoming 

 more complete, until the individual 

 cells are no longer capable of life 

 separated from the rest. The colony 

 of more or less independent individuals would then have given 

 place gradually to a single multicellular organism, in which the 

 individualism of the separate cells has been subordinated to 

 the life of the whole, thus rendering possible the higher type 

 of the multicellular organism, such as we see exemplified in 

 the fresh-water Hydra, described in the next chapter. 



Classification of the Protozoa mentioned in Chapter I. 



All these unicellular forms, whether they live isolated or in 

 colonies, are included in the lowest phylum of the animal 

 kingdom, the PROTOZOA. They are all clearly marked off from 

 other animals by their unicellular structure. The group is a very 

 large and varied one, and it is therefore divided into several 

 classes for convenience in studying, and these classes are again sub- 

 divided into sub- classes or orders. The forms mentioned in this 

 chapter belong to four different orders. 



Amongst the Protozoa all those with blunt finger- 

 ^e processes of protoplasm, like the pseudopodia of 

 Amoeba, are included in the order Lobosa. 



Those similar to the organisms whose shells form 

 chalk rock, are known as the Foraminifera, for in 

 them the protoplasm is enclosed in a calcareous shell, 



Lb s 1 ' 



Order 2. 



Foramini- 



fera. 



1 For further details see Parker's Biology, Lesson xii. 



