68 



colony's composition of one or more difTerent, individual forms, while the last 

 are those which are found in the structure of liie single zoo'cia. 



Colonial form and mode of growth. Throughout the organic world wherever 

 single individuals are uniteil into colonies or in florescences we lind a repetition 

 of the same colonial forms oi' forms of growth. The colonies may he incrusling 

 or free foliaceous or branched in various ways, the single indi\i(hials arranged 

 in one layer or two, in one, two or more rows etc., and this harmony in Ilie 

 outer arrangement may often produce a surprising likeness between animal forms 

 very different in slructure, occasionally even between certain animal and plant 

 forms. It is therefore easy to understand that the lirsl investigators of the numer- 

 ous aggregate animals of the sea, the single individuals of which only reveal 

 their peculiarities on very close examination, have tried to arrange this varie- 

 gated multitude after likeness in the colonial form. We may for example refer to 

 Ellis' celebrated work on the Corallines', under which common name be not 

 only classes hyilroid polyps, Bvijozoa and corals, but also certain calcareous algae. 

 By and by as knowledge of the single individuals of the colonies advanced, the 

 systematic importance of the colonial form becomes more and more limited, as 

 it is gradually used for less and less extensive systemalic units, and in the pre- 

 sent day /Jryoroa system, which is founded on Smitt's and Hincks' well-known 

 works, it occupies a very subordinate position. As there is nevertheless too much 

 importance still attached to the colonial form as systematic character, not only 

 within the Bryozoa, but also within other aggregate animals, for instance the 

 hydroid polyps, I do not think it unnecessary to discuss this question here, and 

 I may first quote some observations concerning this made by Hincks-. After 

 having spoken about the slight hel|), which the j)olypide, and the avicularia 

 give us in systematic regards, he says: »There remain the characters of the cell 

 itself and the habit of growth. It can hardly be deemed doubtful which of Ihem 

 should have the |)recedence in a natural system; we may go very much furlhei-, 

 indeed, and say llial in such a system the latter must hold a very secondary 

 and subordinate place. The essential structure of the cell, as one of the primary 

 zoa'cial forms, must certainly be accounted the most imj)ortant point, holh in it- 

 self and as a clue to relationship. The mere habit is, so to speak, a suj)erinduced 

 condition, which may be difTerent in the most nearly related and similar in the 

 most divergent forms; and groups based on it, instead of fitting in with natural 

 affinities, are found to traverse them at all points*. A little further on' he also 

 slates: »In the Escharine group it seems to me that the families and genera 



r.' ' •>•>. Introtiiiclioii. p CXXVIII, ' p CXXX. 



