284 Proceedings of the Royal Physical Society. 
externally by the ccenosarc [polypary], thus reminding us 
of the sclerobasic corallum of some of the Actinozoa." This 
doctrine had been previously 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 
Tubulariadse, 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 polypidom 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 following conclusions, after the examina- 
tion of a very large number of specimens, some hatched 
from the egg and adherent to glass, others removed as cut- 
tings from adult specimens and transplanted on glass, to 
which they readily 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 Clavo. repens 
(mihi), the (7. discreta of AUman.) 
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 
G. membranacea (mihi), formed from states 1 or 2 by the 
continual fiUing-up of the meshes by anastomosing branches, 
with or without spines. 
4. A fleshy plate of ectoderm permeated by a network of 
