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



branches, which come ofif at right angles and immediately divide into a large number 

 of divergent twigs which bear the polyps at their ends. Each twig bears, on an average, 

 five polyps. Owing to the regular ramification each secondary branch, as well as the 

 smaller main branches, forms a short, convex umbel. All the umbels are crowded 

 together so that each branch appears to be covered with a continuous coat of polyps. 

 The polyp umbels of the four main branches themselves are likewise crowded together. 

 The area of each only appears defined by a groove, owing to the different development 

 as regards length, so that the areas of the branches correspond to four convex pro- 

 tuberances. The polyp heads, each of which appears distinctly isolated on the surface, 

 are placed singly on short peduncles, 1 to 0"8 mm. in length ; the older ones on a twig, 

 being usually more prominent than the others. They diverge somewhat from one 

 another, and are sur-^ounted each by a rigid spicule for a distance of 0*5 to 0"8 mm., 

 upon which they appear to be laterally placed. The diameter of the head reaches 

 0-8 mm. 



The twigs, like the polyp heads, are usually stiS" and rigid, owing to the numerous 

 calcareous spicules embedded in the mesoderm. More especially may be noticed amongst 

 these a band of large spindles which occupies the portion of the twig and of the polyp 

 peduncle, which is turned away from the head. The spicule which surmounts each head 

 projects from amongst these spicules. The mesoderm of the barren part of the stem, 

 which is penetrated by very wide canals, contains numerous spicules of special shape, 

 easily distinguishable from those of the preceding species. 



The following are the principal forms which occur: — (1) Small spicules, usually in 

 the form of quadriradiate structures, sometimes in the form of an oblique cross, some- 

 times of more irregular shape, and covered with spines, warts, or jagged branching 

 processes. Size, 0-2 by O'lS ; 0-15 by O'U ; 0-12 by 0-08 mm. (2) /-shaped spindles, 

 thickly covered with short, branched warts. Size, I'O by 0"13 mm. (3) Simple, curved 

 spindles, covered with sharp spines, which bear in the centre, where they are broadest, 

 branched, thorny processes; length, 0'51 by 0"1 mm. (4) Club-shaped structures, 

 0"31 mm. long and O'l mm. broad at the thickened end, where they bear branched 

 spines. 



In the polyp-bearing portion there occur, in place of these structures, straight, slender 

 spindles, covered with fine spines, or such as are slightly curved or /-shaped. In the 

 stem and main branches they are chiefly placed transversely ; in the twigs they form 

 longitudinal bands, of which one, on the dorsal side of the twig and of the peduncle, is 

 especially developed and distinguished by the great development of the spicules. Here the 

 spicules are 3 to 4 mm. long, with a diameter of 0"2 mm. Their surface is covered with 

 very fine spines, and one end is often shortly bifurcate. The smaller spicules have the 

 following dimensions — length to thickness, 1 by 0"07 mm.; 0'92 by 0'04 mm. 



The spicules of the polyp heads are usually arranged en chevron in eight series; 



