GEMMARIA TMPLEXA. 291 



GONOSOME. — Planoblasts shortly pedunculate, forming a more or less dense 

 cluster, which surrounds the body of the hydranth in a plane which is much nearer to 

 the proximal tlian to the distal limit of the tentacles ; manubrium of medusa sub- 

 cylindrical, extending through about one half the vertical diameter of the umbrella- 

 cavity, receptacles of thread-cells in umbrella very superficial, extending from the 

 circular canal over about one quarter of the entire height of the umbrella ; thread- 

 cell-bearing sacs of tentacles oval, carrying a pencil of long vibratile cilia on their 

 free ends, and borne on very extensile peduncles. 



Colour. — Hydranths white, with the gastric cavity very pale pink, perisarc on proximal 

 ])ortion of hydrocaulus brown, on distal portion colourless. Manubrium and tentacular bulbs of 

 medusa brownish red. 



Bevelopmenf of Gonosome. — April to June. 



Habitat. — On stones in tide-pools, and on old shells, &c., from deep water. 



Btttltijmetrkal Distribution. — Litoral to deep water zone. 



Localities.— Coast of Northumberland, Mr. Howse, Mr. Alder, and Mr. Ilodge ; Firth of 

 Forth, Dr. T. S. Wright and G. J. A. ; Coast of Forfarshire, G. J. A. 



The short hydrocaulus and large many-tentacled hydranth, with its body long and cylin- 

 drical or short and thickly clubshaped, according to its state of extension or contraction, con- 

 stitute marked features in this interesting hydroid. In every instance I found that those 

 Hydranths which carry gonophores have the body shorter and the tentacles less developed than 

 in those from which no gonophores are developed, but the arrest is comparatively slight, and in 

 no case can it be regarded as reducing the hydranth to the condition of a blastostyle. 



Alder describes the perisarc of the hydrocaulus as consisting of two distinct coats, separated 

 by an interval from one another, the inner one annulated, the outer smootii. Nothing of this 

 kind was seen in any of my specimens, in which the perisarc always became very thin and trans- 

 parent for some distance below the hydranth, where it formed a wide tube, including a consider- 

 able interval between its walls and the coenosarc which occupied its axis. This interval was 

 traversed by numerous regularly disposed transverse ileshy processes, which radiated from the 

 coenosarc and became attached to the inner surface of the perisarc. 



It would seem from Mr. Alder's description that the hydrocaulus is sometimes much more 

 developed than in the specimen here figured. I have occasionally found specimens with a some- 

 what longer hydrocaulus, which might even send off a branch, but the difference was never so 

 great as to justify our assigning to it a value greater than that of an unimportant variation. 



The planoblasts have been described at length in a former part of this Monograph (Part I, 

 p. 224), and it is these which will especially arrest the attention of the observer, whether he 

 regards the curious piriform sacs filled with thread-cells which lie one over each radiating canal, or 

 the pedunculated sacs, also loaded with thread-cells, which are borne in multitudes along the 

 entire length of the marginal tentacles. The peduncles of these sacs are singularly extensile, 

 and may sometimes be seen stretched out to a great length, carrying the sacs on their extremities, 

 and giving to the tentacles the appearance under the naked eye of being covered with a dense 

 growth of some parasitic mould. On being touched they immediately contract, and become 



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