CVl INTRODUCTION. 



Special Points. — Affinities. 



i. Relation of Zooecium and Polypide. — As we have seen, 

 eacli element of the Polyzoan colony consists of two parts, 

 the cell and polypide. We have to consider briefly how 

 these two are related to each other. Are we to regard 

 them as constituting together an individual ? or must we 

 call them two zooids, associated, but individually distinct ? 

 According to the former view (which is the older), the 

 polypide is an organ or collection of organs, the cell the 

 enveloping sac, and their relation is the same in kind as 

 that between any two ordinary portions of structure. 

 Allman was the first to hold the true zooidal nature of 

 the two parts; to him both the zooecium and polypide 

 are individuals, the latter produced by budding from the 

 former; and to this extent his doctrine has been gene- 

 rally accepted. But he pushed it still further, and con- 

 tended that the ovary and testis, also produced by in- 

 ternal gemmation, are equally individuals, though of a 

 humbler type. Nitsche * has pointed out the objec- 

 tions to this interpretation, and has insisted especially on 

 the difficulty in the way of regarding the loose aggrega- 

 tion of sperm-cells, which constitutes the testicle, as the 

 homologue of a distinct individual f- 



We have no difficulty in recognizing the zooidal nature 

 o£ the zooecium, when we follow its history, and find it 

 able to live apart from the polypide, and even to survive 

 several generations of polypides derived from its sub- 

 stance. Besides we meet with it under various forms, in 

 some of which it is never associated with a polypide or 



* ' Morphologie d. Bi'yozoen,' Zeitsch. &c. xxi. 4 Heft, p. 100. 



t Hatschek disputes the zooidal nature of the zoa?cium and polypide ; 

 Repiachoff also rejects the doctrine of the distinct individuality of the latter : 

 Zeitsch. &c. xsvi. 



