70 
THE VOYAGE OF H.M.S. CHALLENGER. 
of the hydranth is fixed within the hydrotheca by bands which stretch from it to the 
walls of the hydrotheca, and it is only the distal end of the hydranth which admits of 
retraction and extension. The tentacular crown, even in its state of extreme contrac- 
tion, is incapable of being withdrawn into the cavity of the hydrotheca, and, notwith- 
standing the complete development of the hydrothecae, the hydranths derive almost as 
little protection from them as those of Halecium do from the hydrothecse in that genus, 
where these receptacles are rudimental. However extensile may be the bands which 
stretch from the body of the hydranth to the walls of the hydrotheca, it would seem 
that they operate in fixing the body of the hydranth and thus preventing the complete 
retraction of the hydranth within the hydrotheca. A condition entirely similar to this 
occurs in Sertularia exserta. See above, p. 56. 
The depth of 770 fathoms from which Thuiaria hyalina was dredged adds to the 
interest of the species, and the transparency and absence of colour in its perisarc has 
probably some relation with the great depth of its habitat. 
Desmoscyphus, Allman. 
Desmoscyphus, Allman, Journ. Linn. Soc. Lond. (Zool.), vol. xii. 
Generic Character. Trophosome. — Colony dendritic ; hydrocaulus divided by joints 
into internodes, each internode corresponding to one or more pairs of hydrothecae. 
Hydrothecae of the ramuli all brought to one side of the ramulus and adnate to it 
by their epicauline walls, adnate also or in close apposition to one another by their 
opposed sides. 
Gonosome. — G-onophores adelocodonic, gonangia destitute of marsupium. 
The genus Desmoscyphus was originally constituted for a Hydroid from New Zealand. 
In the extent to which the hydrothecae are adnate to the hydrocaulus it agrees with 
many species of Thuiaria, while another point of agreement with that genus will be 
found in the fact that in some parts of the colony a single internode may carry many 
hydrothecae. From Thuiaria, however, it is obviously separated by the hydrothecae on 
the ramuli being all brought to one side of the ramulus, where they become in almost 
every instance adnate to one another in pairs along their opposed sides. In Desmoscyphus 
pectinatus, one of the species obtained by the Challenger, while the hydrothecae are closely 
approximate, their walls "have not actually coalesced with one another, though the 
hydrothecae present the essential character of the genus in being all brought to one side 
of the branch, instead of being distichous as in Thuiaria and Sertularia. 
It is only in the branches that the characteristic condition of the hydrothecae is 
constant. In the main stem, especially towards its proximal end, the hydrothecae may 
recede from one another and ultimately become disposed in two opposite series, separated 
as in Thuiaria and Sertularia by the entire width of the stem. 
