INTRODUCTION. CVll 



any representative of one. The internodes of the stem 

 amongst the Ctenostomata are clearly homologous with the 

 zocEcia ; they are cells living as simple zooids. In the 

 gonoecium we have the same morphological element dis- 

 charging reproductive functions, without any help from 

 the polypide ^. The latter^ on the contrary, seems to be 

 always in connexion with a zooecium ; but the history of 

 the cell shows clearly that we are not to regard the poly- 

 pide as its organ, or in any sense an essential part of it. 

 We can only, therefore, view it as an associated zooid ; 

 its own structure and habits forcibly suggest the same 

 interpretation. An animal periodically losing all its 

 internal organs and as regularly reproducing them would, 

 indeed, be an anomaly. We are, then, to regard the 

 zooecium and the polypide as two individuals, one perma- 

 nent, the other ephemeral, but capable of renewal, which 

 compose together the primary element of the Polyzoan 

 colony (the '' polypocystid " of Nitsche)t. 



ii. Excretonj Organs. — It should be noted that very 

 simple excretory organs occur in some of the Polyzoa, 

 corresponding with those of worms, other Mollusca, &c. 

 These are the nephridia (primitive kidneys) of Ray Lankes- 

 ter; and consist of minute ciliated tubes, passing from 

 the body-cavity to the exterior, and forming a medium 

 of communication between the two ; they open out in the 

 neighbourhood of the anus. They were first detected 

 by Hatschek j, in both the larval and adult forms of 

 Pedicellina, and subsequently by Joliet § in Loxosoma. 



* The case of the gonoecium may be cited against the opinion maintained 

 by Joliet, that the polypide and not the zooecium (as commonly held) is 

 the sexual zooid {op. cit. pp. 83, 84). 



t The avicularium and Tibraculum must also be accounted distinct zooids. 



\ Zeitschrift, &c. xxix. (1877), 4 Heft, p. 51G. 



§ Comptes Eendus. Ixxxviii. (1879). 



