THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 95 



the individuality made up of them. In nearly all the forms 

 indicated, the mutual dependence of the united animals is so 

 Blight, that they are more fitly comparable to societies, of 

 which the members co-operate in securing certain common 

 benefits. There is scarcely any specialization of functions 

 among them. Only in the last type described do we see a 

 number of individuals so completely combined as to simulate 

 a single individual. And even here, though there appears to 

 be an intimate community of nutrition, there is no physio- 

 logical integration beyond that implied in several mouths and 

 stomachs having a common vent.* 



§ 204. We come now to an extremely interesting question. 

 Does there exist in other sub-kingdoms composition of the 

 third degree, analogous to that which we have found so 

 prevalent among the Ccelenterata and the Polyzoa and Tuni- 

 cataf The question is not whether elsewhere there are 

 tertiary aggregates produced by the branching or clustering 

 of secondary aggregates, in ways like those above traced; 

 but whether elsewhere there are aggregates which, though 

 otherwise unlike in the arrangement of their parts, never- 

 theless consist of parts so similar to one another -that we 

 may suspect them to be united secondary aggregates. The 

 various compound types above described, in which the united 

 animals maintain their individualities so distinctly that the 

 individuality of the aggregate remains vague, are constructed 

 in such ways that the united animals carry on their several 

 activities with scarcely any mutual hindrance. The members 

 of a branched Hydrozoon, such as is shown in Fig. 149 or 

 Fig. 150, are so placed that they can all spread their tentacles 

 and catch their prey as well as though separately attached to 

 stones or weeds. Packed side by side on a flat surface or 



* It has been pointed out that I have here understated the evidence of 

 physiological integration. An instance of it among Hydrozoa is shown in 

 Fig. 151, but by a strange oversight I have forgotten to name the various 

 cases furnished by the Siphonophora in which the individual polypes of a com- 

 pound aggregate are greatly specialized in adaptation to different functions. 



