PENNATULIDA DREDGED BY H.M.S. "TRITON." 127 



The agreement will be seen to be absolute, even as regards the actual size 

 and arrangement of the spicules, which in both cases are far larger and more 

 abundant on the abaxial or outer surface than on the axial or inner one. It 

 is also worthy of note, in connection with the point in question, that in the 

 development of the polyps the calyx processes appear earlier than the 

 tentacles (vide fig. 7). 



The large zooids of P. phosphorea var. aculeata agree, therefore, with the 

 zooids of Pennatulida generally in the complete absence of tentacles, as well as 

 in the absence of reproductive organs and the possession of but two mesenterial 

 filaments. They are peculiar merely in their very great absolute size, and in 

 the prolongation of the abaxial surface to form the spine. 



The structure of one of the normal small zooids is shown in fig. 8, e. It will 

 be noticed that here also the mouth, which in the early stages of development is 

 terminal (as shown in the rudimentary zooid between e and the large zooid), 

 becomes thrown over to the axial surface by growth forwards of the abaxial 

 side, which forms a prominence above the mouth clearly comparable to the 

 spine of the larger zooids. The figure shows also that the smaller zooids, like 

 the large ones, possess the clothing of exceptionally long cilia on the abaxial 

 surface of the stomodseum (siphonoglyphe of Hickson). 



The small zooid in question (fig. 8, e) is an immature one, as there is as yet 

 no communication between the stomodseum and the body cavity of the zooid; 

 the septa and mesenterial filaments have also not yet appeared. 



Panceri # has described and figured an interesting abnormality occurring in 

 a specimen of P. phosphorea, in which four of the latero-yentral zooids, three 

 on the left side of the rachis and one on the right, have the form and structure 

 of fully developed polyps, inserted independently into the rachis, and attaining 

 a length of 10 mm. and a diameter of 1 to 2 mm. In describing this curious 

 modification, Panceri discusses briefly the mutual relations of polyps and zooids, 

 points out the fundamental and essential correspondence between the two, and 

 infers that the zooids must be regarded as abortive polyps, and that such cases 

 as the one he describes are to be viewed as examples of reversion from the 

 abortive to the fully developed condition. 



In this view Panceri is undoubtedly right. In a colony of individuals 

 formed by continuous gemmation, i.e., by a process of budding in which the 

 several zooids remain organically connected together to form the colony, the 

 several component individuals must be supposed to be primitively all alike and 

 equivalent to one another. Differences in structure and function could clearly 

 only have arisen after the habit of forming colonies had been established for 

 some time. Hence those colonies will be the most primitive in which there is 



* Panceri, " Intorno ad una forma non per anco notata negli zooidi delle pennatule," Rendiconto 

 delVAcademia delle scienze fisiche e matematiche, pp. 23-28, Napoli, Febbrajo, 1870. 



VOL. XXXII. PART I. Y 



