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



should be included within the family Nephthyidse, for, in spite of an external appearance 

 widely different from Spongodes and Nephthya, which is expressed in the Gorgonid-like 

 habit, the inner structure of the colony corresponds closely with that of the above- 

 mentioned family. Its higher and more slender growth necessitates an increase in the 

 strength of the colony ; this has been given to it by means of a considerable develop- 

 ment of its spicular network in the walls of the polyp tubes. 



As in Spongodes, the branches and twigs consist of bundles of polyp tubes, four, 

 or sometimes five, in number, terminating in the polyp heads at the end of each 

 twig. 



The tubes, consisting of thick mesodermic walls, communicate with each other by 

 means of delicate endodermic nutritive canals. The walls of the tubes meet in the 

 axes of the twigs so that the individual digestive cavities of the polyps are arranged around 

 a kind of axis in a radial fashion. Fresh buds from the sides of the polyp walls arise 

 from the system of the nutritive canals, so that in cross section their digestive cavities 

 are always exterior to the four main canals. Also there may be distinguished, in the 

 larger stems, four or five broader tubes in the centre, separated from one another by 

 thick ccenenchyma. This investing ccenenchymatous layer is filled with large spicules 

 arranged parallel to the longitudinal axis. To the outside of these there are additional 

 longitudinal canals of various diameters, which communicate with one another, as also 

 with the broader central canals, in a network of nutritive canals. To the outside of this 

 network is a continuous layer of these longitudinally arranged spicules (see PI. VI. fig. 5). 



From without inwards, then, the stem consists of — 



( 1 ) Ectoderm, as a thin layer of flattened cells. 



(2) A layer of large spicules. 



(3) A circle of polyp tubes. 



(4) Thick coenenchyma containing large spicules. 



(5) Four or five wide central canals, the walls of which are in contact in the 



centre. 



The polyps, in the greater number of species, consist of a calycine portion into which 

 the alimentary and tentacular parts of the polyp may be withdrawn. On the tentacular 

 portion is a collar of spicules arranged in a circle, and, outside of these, spicules, arranged 

 en chevron, which strengthen the base of the tentacle. The spicules are continued up 

 to the apex of the tentacle. In rest the tentacles are folded together over the mouth of 

 the polyp. 



In one species, Siplionogorgia pendula, n. sp., the anterior portion of the polyp can 

 be withdrawn into the bilateral calyx only as far as the collar ; the oral region, with the 

 spicule-bearing and folded tentacles, remains outside the calyx. In this respect the form 

 of the polyps agrees entirely with that of the polyps in Spongodes. 



