10 THE VOYAGE OF H.M.S. CHALLENGER. 
metrically disposed on the walls of the gonangium. Over each of these points a nemato- 
phore is developed externally, while a perforation in the chitinous walls allows of the 
free communication of the contents of the nematophore with the subjacent branch of the 
blastostyle. 
In the only two known species of phylactocarpal Eleutheroplea the nematophores are 
developed in abundance along the sides of the appendages, which combine to form the 
peculiar phylactocarps of these Hydroids.’ 
That the nematophores play a very important part in the economy of the Plumularide 
would appear from the fact of their early appearance and of their persistence. In 
the development of the trophosome they are in many cases, if not in all, the first 
zooids of the colony to make their appearance, preceding the hydranths and hydrothece ; 
while, as we shall afterwards see, they remain behind in certain cases in which the 
hydranth and hydrotheca, which would normally belong to them, have become entirely 
suppressed. 
I have endeavoured to show elsewhere’ that their phylogenetic or paleeontological 
significance is scarcely less important, for I believe we have good grounds for maintaining 
that the ancient graptolites represent hydroid organisms, in which the nematophores were 
the only zooids of the trophosome which had been developed. 
Morphology of the Gonosome in the Phylactocarpal Plumularide. 
The gonosome in the phylactocarpal forms of the Plumularidee is full of interest, pre- 
senting as it does some significant morphological facts, and affording instructive examples 
of the extent to which variation of form may exist consistently with the retention of 
homological identity. 
The term phylactocarp may be used as a sufficiently convenient general expression for 
the various forms under which the apparatus destined for the protection of the gonangia 
shows itself in the phylactocarpal Plumularide. This apparatus is referrible to one or 
other of two types: it consists either of one or more variously modified hydrocladia, or 
of a separate appendage to the hydrocladium quite distinct from the hydrocladium itself. 
The commonest and longest known form of phylactocarp is the corbula of Aglaophenia 
(Pls. XI. and XII.). This consists of a peculiarly modified hydrocladium, which supports 
on each side a row of ribs or coste in the form of flattened leaf-like appendages, and 
between these two rows a row of gonangia. ‘The leaflets of each side arch over the 
gonangia, and enclose them in a basket-shaped receptacle. They are furnished with 
numerous short teeth-like nematophores on one or both edges, recalling somewhat the 
' See J. W. Fewkes, Bull. Mus. Comp. Zool., loc. cit. 
° Gymnoblastic Hydroids, p. 179. 
