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



two dorsal folds remain longer demonstrable. The mesoderm wall of the polyps becomes 

 thicker from the apex towards the base, while the diameter of the gastral cavity 

 remains always the same. In the oesophageal region one can distinguish in the upper 

 part, outside the endoderm, a thin structureless layer which is continued into the 

 mesenteric folds. Outside of this lies the gelatinous layer which contains the spicules. 

 In decalcified preparations the sheaths of the spicules may be recognised, sometimes with 

 a trace of the nucleus of the spicule-producing cell. Below the tentacles narrow 

 endodermic canals may be seen to penetrate ; they ramify in the mesoderm, and are 

 connected by fine processes with the gastral cavity of the polyp. In the oesophageal 

 portion these nutrient canals dilate to form lacunae, Avhich in part fuse together (fig. 6) 

 and finally unite further down into longitudinal canals continued on (fig. 7) towards the 

 base. By the increasing thickness of the mesoderm these canals are pressed more and 

 more towards the periphery', while round the central canal the spicules become aggregated 

 to form a sort of axis, which is not, however, definitely limited externally. Between the 

 aggregated spicules a few lacunse and fine nutrient canals may still be seen. Towards 

 the base the longitudinal canals attain a width which in part exceeds that of the gastral 

 cavity, and at the same time the spicule- containing mesoderm acquires a more considerable 

 thickness. The central canal can then be distinguished with difficulty, and only by 

 its position from the other canals. A cross section of the main axis presents a sieve- 

 like appearance, and reminds one in some degree of the cross section of Paragorgia. 

 The canals are continued into the basal expansion, and there exhibit an irregular 

 network. 



The axial polyps of the second and third order exhibit a structure quite analogous 

 with that of the above. At their base, however, they are not proportionately thickened, 

 but remain approximately cylindrical. They spring from the lacunae in the wall of the 

 axial polyps of the first order. 



The lateral polyps are club-shaped, 3 mm. in length, at their origin 1 mm. in 

 thickness, but increasing at their ends to 1*2 to 1"5 mm. (fig. 3). They arise at obtuse 

 or at right angles from the axial polyps. Occurring at intervals of 1 to 1 "5 mm. from 

 one another, they form spirals in which the fifth polyp comes to lie over the first. Their 

 structure (fig. 5) is essentially the same as that of the axial polyps. The tentacles again 

 appear as simply folded together over the invaginated oral region. The oesophageal tube 

 narrows suddenly below, but the narrow tube into which it is continued is continued 

 laterally on the base of the wider portion, and that on the side turned towards the stem, 

 so that the polyp acquires a bilateral appearance. The gastral cavity is short, and is 

 connected by means of canals, which spring from the base, with the canalicular system 

 of the axial polyps. In the wall of the polyp also nutrient canals are distributed, and 

 the polyp may by longitudinal growth and production of buds develop into an axial 

 polyp. 



