98 THE VOYAGE OF H.M.S. CHALLENGER. 
pores divided into two chambers, an upper and a lower, by a constriction of their 
walls. Opening between the chambers rendered horse-shoe shaped by the projection 
across it, in the direction. of the tips of the branches, from that side of its margin 
placed nearest the base of the branches, of a tongue-like excrescence. Ampulle in 
the male stocks in a ring around the cyclo-system masses ; none scattered on the 
branches. Dactylozooids, when at rest, doubled down within the upper chambers of the 
gastropores. Gastrozooids flask-shaped, devoid of tentacles, with numerous basal canals. 
Gonangia in the male stocks containing a central mass of cells from the surface of which 
are developed as buds numerous pedicellate lobular sacs, in which the spermatozoa are 
produced. Female stocks unknown. 
12. Cryptohelia, Milne-Edwards and Haime. 
Coenosteum closely resembling that of Astylus in all respects, excepting that the cyclo- 
system masses are not so prominent, that the opening between the upper and lower 
chambers of the gastropores is circular in outline, and that a lid-hke lamina of calcareous 
matter is: directed horizontally across the mouths of all the cyclo-systems. The lids are 
supported on stout columns arising from the margins of the cyclo-systems and inclined 
over them. They spring from the sides of the systems nearest the bases of the branches, 
and are directed towards the tips of the branches. In female stocks only a single 
ampulla and gonangium developed in relation with each cyclo-system. No ampulle on 
the connecting branches. In the males several ampullee in the walls of each cyclo- 
system. Soft structures as in Astylus. In female stocks numerous gonophores present 
in each gonangium in all stages of development. Spadix cup-shaped, developing, as 
in Errina, into a fringed network at the margin. A solitary ovum developed in relation 
with each spadix. Planula very long and worm-like. 
PEDIGREE OF THE HyDROCORALLIN2E. 
The line of descent of the various genera of the Stylasteride from a parent form seems 
to be traceable with especial clearness. All gradations are present by which simple 
circular mouthed pores sporadically scattered over the coenosteum become grouped and 
modified into cyclo-systems of the most symmetrical and complex character. Since styles 
appear in some genera in the dactylopores as well as in the gastropores, it seems probable 
that in the ancestral form or “Archistylaster” styles were present in both forms of pore. 
If the Milleporidee prove closely related to the Stylasteridee when thew gonophores 
have been investigated, it will follow that the two families have had a common ancestor, 
and that Hydroids have developed a calcareous support only once in their history 
and not in two separate instances. This common ancestor may be presumed to have 
had a hydrosoma composed as throughout the sub-order Hydrocoralline ; with its 
