18 Edmund ß. Wilson 



over, the Oesophagus extends far downwards, so that the dorsal fila- 

 ments , even when very short , extend nearly to the base of the bud. 

 Hence the upward currents are unnecessary until the bud elongates con- 

 siderably, and the ectodermic bands therefore elongate only in later 

 stages. 



In Gorgonia the canal-system is well-developed, but the Oesopha- 

 gus extends even farther inwards than in Paralcyonium. Hence in this 

 case also the upward currents are at first unnecessary and the elon- 

 gation of the filameuts takes place only in later stages. 



Turning now to the zooids of the Pennatulida we find similar 

 conditions. The general rule is that they communicate by their 

 bases with the canal-system, and the dorsal filaments, when present, 

 extend to the bottom of the stomach-cavity, sometimes attaining an 

 euormous development. If my hypothesis be correct, we should expect 

 to find in those zooids which do not possess the filaments, special phy- 

 siological conditions which obviate the need for definite upward cur- 

 rents. I bave myself investigated only one such case — the zooids 

 namely oiBenilla — and we can bere clearly perceive such conditions. 

 The zooids of Äe we7^a are not elong ated and open directly 

 into the cavities of the feeding polyps without the inter- 

 position of a canal-system 1, so that as in the young bud of P«/-- 

 alcyonium the fluids can pass directly into the zooid. 



For the zooids of other Pennatulida I must refer to Kölliker's 

 great work, in which however the anatomical relations are not always 

 described with sufficient minuteness to clear up the present point. 

 Looking through bis descriptions we find that the zooids can be thus 

 grouped : 



A. Zooids with filaments. 



Pteroides (zooids of the »Zooidplatte«) 

 Pennatula Funiculina 



Ptilosarcus Kophohelemnon 



Halisceptrum Policella 



Pavonaria Veretillum 



Halipteris Stylohelemnon 



B. Zooids without filaments. 



Pteroides (Ventral zooids , zooids of the »Kiel« and of the upper face of 

 the leaves] . 



1 See also Kölliker's Pennatuliden, p. 273. 



