506 G. H. PARKER 



type and, therefere, capable of transmitting in its own plane 

 from any point as a center radially in all directions. But, how- 

 ever transmission may be explained, the luminous response of 

 Renilla belongs to that category of reactions that involves the 

 organization of Renilla as a whole and, though the transmission 

 is obviously of a diffuse character, the luminous response is much 

 more indicative of colonial unity than, for instance, the action 

 of the autozooids is. 



The relations pointed out in, this paper are not without a cer- 

 tain morphological interest. The unit of structure of such a 

 colony as that of Renilla is quite obviously the zooid. Each 

 zooid is made up of cells combined into tissue and these into 

 organs. Thus each zooid exhibits a series of graded relations 

 that are also characteristic of any metazoan individual. It has 

 long been recognized that most protozoans are unicellular and 

 hence cannot be said in any proper sense to have tissues or or- 

 gans, for these are always formed by combinations of cells. It 

 is obvious, however, that the single protozoan cell often has 

 special parts that perform particular functions in precisely the 

 same way that the organs of metazoans do. As these parts can- 

 not be properly designated as organs, they have been termed 

 by some organellae. If it is inappropriate to speak of organs 

 in protozoans because this term should be restricted to the mul- 

 ticellular parts of the metazoan individual, it is also inappro- 

 priate to use it in reference to a structure in a metazoan colony, 

 even though it may there perform a special function. Thus 

 while it is quite appropriate to designate the tentacle of a zo5id 

 in Renilla as an organ, for it is a multicellular functional unit 

 in a single individual, it is not appropriate to speak of the pe- 

 duncle of Renilla as an organ, for this is a structure that serves 

 the whole colony of individuals. Such structures stand above 

 ordinary organs as organs stand above organellae. They might, 

 . therefore, be called superorgans. In Renilla they are repre- 

 sented not only by the peduncle as a structure concerned with 

 the inflation of the colony as a whole, but by the nerve-net that 

 controls colonial luminosity. Superorgans give a unity to a 

 colony that is often unexpressed in the individuals of which it 

 is composed. 



