British Protozoa and Zoophytes. 131 



solid chitinous polypary [polypidom] is covered externally by 

 the coenosarc [polypary], ^ 1US reminding us of the sclerobasic 

 corallum of some of the Actiuozoa." This doctrine had been pre- 

 viously promulgated by Quatrefages (Ann. des Sc. Nat. xx. 232), 

 who considered the polypidom to be an endoskeleton deposited 

 in the substance of the polypary, like the solid axis of Gorgonia. 

 If this view were correct, it would not only remove Hydractinia 

 from the Tubulariada?, but would segregate it from the whole 

 of the Hydroid Zoophytes, not one of which is destitute of an 

 investing polypidom. 



In the ' Edinb. Phil. Journal ' for April 1857, I stated, in a 

 paper on Hydractinia, my conviction of the incorrectness of 

 Quatrefages's opinion, and that the mode of secretion of the poly- 

 pidom of Hydractinia did not differ from that of the rest of the 

 Tubulariadse, as was seen in the development of its young and 

 its propagation by stolons. Since then I have come to the fol- 

 lowing conclusions, after the examination of a very large number 

 of specimens, some hatched from the egg and adherent to glass, 

 others removed as cuttings from adult specimens and trans- 

 planted on glass, to which they 1'eadily grow, and others removed 

 entire from the shell of the Pagurus by acid, and put up in spirit 

 or balsam. 



The polypidom and polypary are found in the following forms, 

 all of which are frequently combined in the same specimen : — 



1. An open network of delicate chitinous tubes without spines, 

 enclosing a polypary composed of several combined endodermal 

 tubes surrounded by a single layer of ectoderm. Found in very 

 young specimens, or in old ones growing on protected parts of 

 the shell. (Analogous to Clava repens (mihi), the V. discreta 

 of All man.) 



2. An open network as in the last; the tubes of thick brown 

 chitine, with single hollow spines rising from a single tube, or 

 from the confluence of four tubes. 



3. A close reticulate plate, as in Clava cornea (mihi) and C. 

 membranacea (mihi), formed from states 1 or 2 by the continual 

 filling-up of the meshes by anastomosing branches, with or with- 

 out spines. 



4. A fleshy plate of ectoderm permeated by a network of 

 endodermal tubes, and covered above and below by a delicate 

 investment of chitine. Found on the growing borders of the 

 zoophyte, and especially in cuttings of old specimens transferred 

 to glass. 



The spines are composed of one tube or many parallel tubes : 

 they may be single (PI. V. fig. 4), and developed on a single tube 

 of the polypidom, like those of Podocoryne fucicola ; single at 

 their summits and of several tubes at their base (figs. 5 & G) ; 



9* 



