CHAPTER V 



COMPARATIVE ORGANOLOGY: OCELLI, PUSULES, 



NEMATOCYSTS 



In ari}^ group of organisms differentiation within the group comes about 

 through modifications of their characters more often than by tlie addition of 

 new ones. In the Protozoa, which in the number of indiA'iduals far exceeds 

 that of any other phyhun, tlie number of structures u]:)on which these changes 

 may be rung is exceedingly small in comparison with the Metazoa. To permit 

 the production of the 'S'ast number of different kinds of protozoan organisms, 

 then, modifications have appeared, manifold in variety, usually minute and often 

 obscure, yet making a grand total that gives to this group a diversity of form 

 and appearance as great as, if not greater, than may be found in any other 

 phylum. 



In all this l)affling arra_v it is no small task to imravel the tangled lines of 

 growth and variations and place the fundamental structures in their jjroper 

 relationships. The task is often rendered further impossible by the lack of 

 adequate data and by misinterpretations of both form and function which ob- 

 scure much of the earlier literature. The work must of necessity, therefore, 

 be only fragmentary, providing a basis upon which future investigation may 

 place the growing structure. 



The fundamental features of the Protozoa consist of motor organelles with 

 their related structures, organelles for food-getting, assimilation and excretion, 

 nuclei and skeletal modifications. Some of these structures, as external motor 

 organelles and nuclei, have less significance from the standpoint of comparative 

 mor])hologv, especially of the smaller groui:)S and of species, since their relations 

 and homologies are obvious and fundamental. The other structures of the 

 protozoan body are subject to greater modifications, and their interrelations 

 become increasingly complex as speciation progresses. 



This is especially' true of those organelles which, present in a simple or 

 modified form in the Protozoa, have been carried on into the Metazoa and find 

 there their highest develoi^ment. Beyond the definite structures directly cor- 

 related with the functions pertaining to the comparatively simple type of organ- 

 ization found in the ordinary cell, structures of this t}'pe are not nmnerous, 

 and have always been the occasion for much skepticism. Among such struc- 

 tures may be mentioned the ocelli and nematocysts of the dinoflagellates, and 

 the nervous and muscular elements found throughout the Protozoa generally, 

 reaching their highest development in the group in the neuromotor apparatus 

 of the higher flagellates and ciliates. It is, however, in these fundajneutal types 

 of living organisms, witli their vast periods of evolutionary development and 



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