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



3. Acanthoisis, n. gen. — Colony ramified in one plane, polyps insignificant, 

 cylindrical, with truncated mouth-opening. Spicules of the calyx minute, 

 needle-like. Toothed ribs on the calcareous joints of the axis. 



Genus 1. Primnoisis, n. gen. 



Colony ramified, consisting of a main stem, which gives off branches from three to 

 four sides, usually arising at varying heights, and giving ofi" abundantly lateral twigs, so 

 that the whole colony forms a thick bush. 



The ccenenchyma is thin and allows the jointed axis to show through. The polyps 

 are isolated on the stem, numerous on the branches and twigs ; in loose to close spiral 

 order. They project either at right angles to their support or their axis is directed 

 obhquely towards the apex of the twig. Their form is cIuId- or cup-shaped, the base 

 broad, the middle more or less constricted, the mouth-opening widened. The tentacles 

 are provided with spicules on the dorsal side, and are folded together in repose inwards 

 over the mouth-opening. The form of the polyps is markedly radial and the mesenterial 

 folds are in great part so filled with calcareous spicules that these remain well preserved 

 and rigid even in dried specimens. 



The cylindrical axis consists in the branches and twigs, of alternating internodes of 

 thick, calcareous material and horny nodes. The calcareous joints are long, in opposition 

 to the very short horny joints, and are provided with distinct longitudinal furrows. The 

 branches begin with a horny joint and arise always from the calcareous joints. Some- 

 times it happens that a branch arises near to the upper edge of the calcareous joint, and 

 that, by thickening of the branch, its fii-st horny joint comes into contact with the 

 horny node of the stem, so that individual branches apj)arently arise from the horny 

 joints ; the terminal joints of the twigs are calcareous. 



The base appears, in the single example where it is preserved, as a flat calcareous 

 lamellar disc, which covers foreign bodies. 



The spicules form two layers in the ccenenchyma and calices. 



The scales of the ccenenchyma are more or less flattened, longish, lancet, rod-like or 

 irregular scale-like bodies covered with warts or spines. Their edges are irregularly 

 indented and serrated, and interlock on opposite sides, so that they form a continuous 

 layer. 



In the polyps there is a deep layer of rod-like indented scales, which are placed 

 in eight longitudinal rows and surround the periphery like so many chains ; above these 

 are flat, transversely placed scales, with toothed edges and warty or thorny surfaces whose 

 edges interlock. These scales are broad and narrow and constitute a firm skeleton for the 

 calyx. In the tentacles two to three rows of scales form the skeleton. The spicules in 



