6 THE VOYAGE OF H.M.S. CHALLENGER. 



In one remarkable genus of Statoplea contained in the Cliallenger collection {Dlp- 

 locheilus (PI. VIII. figs. 4-7), the hydrotheca presents a double margin formed by the 

 development of an external chitinous sheath round the upper third of the hydrotheca. 



Nematophores. — The presence of nematophores. whether fixed or moveable, constitutes 

 the most characteristic feature in the Plumularidee. The nematophores are hollow 

 chitinous bodies, more or less cup-shaped or tulmlar, and having their cavity in communi- 

 cation with that of various parts either of the trophosome or of the gonosome. Their 

 contents consist of protoplasm, in which thread-cells are often immersed, and which has 

 the faculty of emitting pseudopodial prolongations which often attain great development. 



Those nematophores which belong to the fixed tj])Q are usually blunt spine-like bodies, 

 with a continuous cavity and a terminal orifice, and with one or sometimes two lateral 

 orifices. The terminal orifice is usually oblique or deeply emarginate. 



The nematophores of the moveable ty]3e are more constant in form. They are funnel- 

 or trumpet-shaped, and never provided with lateral orifices. They are mostly bi-thalamic, 

 their cavity being divided into a proximal narrow chamber and a distal wide cup- 

 shaped chamber, separated from one another by a transverse perforated septum. The 

 distal chamber opens by a wide terminal orifice, which is usually deeply emarginate on 

 one side. 



Nematophores are either hydroeladial, hydrocauline, or gonosomal, according to the 

 special part of the colony with which they are in immediate connection. An accurate 

 knowledge of their arrangement and modifications will be best acquired by examining 

 them separately in the two great Plumularian sections.^ 



' Bodies resembling the nematophores of the Plumiilaridaj have, however, been found in other Hydroids" 

 Hincks {he. cit., p. 230, pis. i. iv. fig. 2) has constituted the genus Ophiodes for a remarkable Hydroid closely 

 allied to Haleciam, in which he had discovered certain singular bodies in the form of long cylindrical, very contractile 

 tentacula-like appendages, each protected at its base by a small chitinous cup, through which it communicates with the 

 coenosarc of the colony, and terminating distally in a spherical capitulum, loaded with thread-cells. They occur here and 

 there on the stem, at a slight distance below the liydrotheca, but are chiefly found scattered on the creeping stolon. 



Bodies closely resembling these have been found by G. 0. Sars {luc. cit., p. 109, pi. iv. figs. 5-8) in a Hydroid which 

 he refers to the genus Ophiodes of Hincks, under the name of Ophiodes parasitica. This determination is not accepted by 

 Hincks, who proposes for Sars's Hydroid the new generic name of Ophionema, believing it to belong to the Plumularidre 

 instead of regarding it with Sars as a near ally of Halecium. Notwithstanding, however, the resemblance of Sars's figures 

 to a Plumularian, it is plain to me that the affinities of his Ophiodes parasitica are really with Halecium, and that the 

 Norwegian zoologist had a true conception of its nature when he referred it to Hincks's genus Ophiodes. The 

 tentacula-like organs in Ophiodes would seem to take the place of the nematophores of the Eleutheroplean Pliumilarida3. 

 They appear, however, to stand on a much higher grade of development than true nematophores, and to have passed 

 out of the condition of mere protoplasm into that of a multi-cellular organ. If this be so we should not be justified 

 in assigning to both the same significance. 



Belonging apparently to the same group of appendages are certain minute bodies which occur in Opdorhiza parvula, 

 a little Hydroid allied to Lafoea, obtained during the United States exploration of the Gulf Stream. They are here in 

 the form of tubular receptacles, which are developed on the hydrorhizal network, and enclose a granular fleshy column, 

 which supports a cluster of thread-cells (Hydroids of Gulf Stream, p. 14, pi. vii. figs. 1-3). In a nearly allied form, 

 Lafocina tenuis, Sars, from the Norwegian seas, the hydrorhiza supports appendages which would seem to be essentially 

 of the same kind (G. 0. Sars, loc. cit., p. 119, pi. v. figs. 1-5). Minute fleshy processes of doubtful significance have also 



