510 G. H. PARKER 
As all these activities show, the Renilla colony is much more of 
a unit than it is an aggregate of parts; its morphological constitu- 
ents, the zodids, have merged their individuality in that of the 
colony. Probably this merger is not so profound as it is in the 
siphonophores, but is it certainly vastly more so than in such a 
sponge colony as Stylotella, in which the individuals are physi- 
ologically quite distinct and apparently only incidentally attached 
—a, state of affairs that is probably reproduced in many of the - 
simple hydrozoan colonies such as Tubularia and the like. 
Where colonial organization is highly developed, as in Renilla, 
many parts of the colony, like the peduncle, the rachis, and the 
general nerve-net, take on functions that apply strictly to the 
colony, and in this sense belong to an order superior to that of 
the colonial unit, the zodid. These relations are not without a 
certain morphological interest. The unit of structure in such a 
colony as Renilla is quite obviously the zodid. Each zoéid is 
made up of cells combined into tissues and these into organs. 
Thus each zodid exhibits a series of graded relations that are 
also characteristic of the ordinary 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 organs, 
for these are always formed by combinations of cells. It is obvi- 
ous, 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. Since these parts cannot be 
designated as organs, they have been termed organellae. If it is 
inappropriate to speak of organs in protozoans because this term 
should be restricted to the multicellular parts of the metazoan 
individual, it is also inappropriate to use it in reference to a 
structure in a metazoan colony, even though it may there per- 
form a special function. Thus while it is quite appropriate to 
designate the tentacle of a zodid in Renilla as an organ, for it is 
a multicellular functional unit in a single individual, it is not 
appropriate to speak of the peduncle of Renilla as an organ, for 
this is a structure that serves the whole colony of zodids. Such 
structures stand above ordinary organs as organs stand above 
organellae. They might, therefore, be called superorgans. In 
