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



when at rest, are but slightly infolded. The ccenenchyma consists of two layers, a, 

 cortical and a medullary portion. The cortex consists of spicules varying from spindle to 

 club-shaped ; it is thick on one face of the axis and contains the polyps. The short 

 alimentary polyp-cavities are sunk into the ccenenchyma, beyond which the anterior 

 cesophageal and tentacular portions project. The medullary portion of the stem exhibits 

 only a very thin cortical layer, which consists of a smooth, delicate ectoderm, and of a 

 very thin layer of mesodermic substance. It contains no polyps. The medullary mass 

 consists of very closely approximated rod-like spicules, which are united by a reticulated 

 mass of horny material. 



The nutritive system is formed of a network of very fine nutritive canals which unite 

 the polyps. The canals pierce the entire thickness of the cortical substance, and finally 

 open into larger vessels, which extend in diminishing numbers around the periphery of 

 the medullary substance, and are also in part embedded in the cortical sheath. On the 

 thinner twigs these longitudiual canals occur only in the polyp-bearing surface of the 

 ccenenchyma; on the thicker branches they occur over the whole periphery. It is 

 probable that the mechanical conditions of the upi-ight growth is not favourable for a 

 flattened expansion of the stem and branches, for they become inroUed on the side which 

 does not bear polyps. It thus happens that in the stem and larger branches the margins 

 will often touch so as to form a hollow tube, while the smaller twigs only exhibit a 

 channelled stem. The near relation of such a colony to that of Sympodium and associated 

 forms is very marked ; there is still the flat expansion of the stem, but this instead of 

 clinging by its lower face to foreign objects, is elevated and grooved so that its under 

 surface becomes the inside of a tubular mass or a furrow. In the upright mode of 

 growth, furthermore, a firmer support is essential, and this is furnished by the better 

 differentiation of the medullary mass. 



3. Semperina, KoUiker, Verhandl. phys. med. Gesch. Wiirzburg, N. F., Bd. ii. p. 9. 



This genus is neaHy related to the last. Here, however, the stem has assumed a 

 more cylindrical form, and the medullary mass becomes more or less the axis of the 

 colony. The axis still remains, however, eccentric, and the polyps, like those of the 

 previous genus, arise predominantly from only one face of the mass. 



4. Suheria, Studer, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, October 1878, 

 p. 666. 



Here the medullary mass is fairly well difl'erentiated, and forms the central axis 

 of a cylindrical stem. The latter is but slightly branched, and gives off polyps on 

 all sides, but most abundantly on the club-like thickened apical portion. The 



