10 THE VOYAGE OF H.M.S. CHALLENGEE, 



metrically disposed on the walls of the gouangium. 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 Plumularidae 

 would appear from the fiict 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 hydrothecse ; 

 while, as we shall afterwards see, they remain behind in certain cases in which the 

 hydranth and hydrotheca, which w^ould normally belong to them, have become entirely 

 suppressed. 



I have endeavoured to show elsewhere - that their phylogenetic or palseontological 

 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 Gononome iwthe Phylactocarpal PlumularidcB. 



The gonosome in the phylactocarpal forms of the Plumularidas is full of interest, pre- 

 senting as it does some significant morphological facts, and afi"ording instructive examples 

 of the extent to which variation of form may exist consistently with the retention of 

 homological identity. 



The term pliylactocarp may be used as a sufiiciently convenient general expression for 

 the various forms under which the apparatus destined for the protection of the gonangia 

 shows itself in the phylactocarpal Plumularidse. 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 hydi'ocladium quite distinct from the hydrocladium itself. 



The commonest and longest known form of phylactocarp is the corbula of Aglaophenia 

 (Pis. XL and XII.). This consists of a peculiarly modified hydrocladium, which supports 

 on each side a row of ribs or costse 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. Zoo!., loc. cit. 

 - Gymnoblastic Hydroids, p. 179. 



