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CHAPTERS ON EVOLUTION. 



each containing a little tenant the individual animals of the colony 

 do not communicate with each other. On the contrary, each 

 member of the sea-mat colony is perfectly distinct from all its neigh- 

 bours, and lives enclosed in its separate domicile. But for the union 

 of its cell-wall with the walls of other cells, each little sea-mat unit 

 is a thoroughly independent being ; and even the so-called " colonial 

 nervous system," which was long believed to connect the members 

 of the fraternity in a common bond of sensitiveness, has been proved 

 to be non-existent. It is highly interesting, therefore, to find that 

 compound animals may, like the zoophytes, possess their individual 

 or component units in close structural harmony and relationship ; or 



FIG. 190. F LUSTRA, OR SEA-MAT. 

 a, natural tize ; b, magnified, showing the cells. 



may, on the other hand, like the sea-mats, exhibit a collection ot 

 animals each of which is thoroughly independent of its neighbours. 

 That such differences have not originated in any haphazard fashion, 

 but that they are a veritable result of the tendencies of development, 

 is readily enough proved. For, whilst each member of the zoophyte- 

 stock is in free and full nutritive co-operation with its neighbours, each 

 " sea-mat " unit preserves within its own cell, not merely a perfect 

 digestive apparatus, but a nervous system, and reproductive or egg- 

 producing organs as well. The independence of the " sea-mat's " 

 members has been accompanied by the development of a much 

 higher organisation than is found in the interdependent zoophyte- 

 units ; although, of course, such a statement of fact still leaves the 

 origin and cause of the independence of the higher " sea-mat " units 

 an open question. But in its manner of growth, the latter colony 

 resembles the zoophyte. Each unit has the power of adding to the 

 colony by the process of budding already described ; whilst each 

 member of the colony possesses, likewise, the power of giving origin 



