*2 THE VOYAGE OF H.M.S. CHALLENGER. 



The elongated spicules wliich form the root-tuft are either smooth or beset with 

 barbs, disposed iu various ways. While the upper end, which is concealed in the 

 parenchyma, always runs out to a simple point, the free inferior extremity usually bears 

 an anchor-like structure, w^hich varies greatly in form and morphological significance. A 

 knob-like terminal swelling may occur, from the sides of which a few prongs, arranged 

 in a whorl, project obliquely upwards and outwards ; or again, four cruciate rays may be 

 present, lying in two typical axes, or rarely two rays in one transverse axis. These rays 

 are bent upwards in hook-like fashion. In the first case we have usually to deal with diacts, 

 iu which the axial-canal cross is usually recognisable on the stalk, at some distance 

 above the anchors, while the anchor-teeth do not exhibit any axial canal (PI. XIV. 

 fig. 5). In the other cases, however, we have to do with pentacts and triacts, in which 

 four cruciate or two curved transverse rays lie in the same plane, with axial canals 

 usually distinctly recognisable (PL III. fig. 23, and PI. LIV. fig. 9). 



The arrangement of the barbs on the anchor-stalk, and the form of the anchor-teeth, 

 are often generically characteristic. In many root-tufts besides the anchor-spicules, 

 simple pointed diacts frequently occur. 



Pleuralia. — The spicules that project markedly from the lateral walls of the Sponge 

 sometimes exhibit an essential similarity^ to the basalia, from which they cannot, indeed, 

 be sharply separated even in regard to position. In Pheronema giganteum, for instance 

 (Pis. XLV., XLVL), bundles of long spicules with round swellings are radially 

 distributed, with tolerable regularity, over the whole outer surface ; and those directed 

 downwards (basalia) differ from those placed in a more lateral position (pleuralia), only 

 in their greater length, slight curvature, and union in a basal tuft. In the Polylophus 

 philippinensis (PL LIV. fig. 1) numerous pleural spicules assist in the formation 

 of a local tuft, by bending down and becoming approximated to the basalia. The 

 outermost termination of these pleuralia exhibit the same anchor-structures as are present 

 in the basalia of the same form. Thus, both in pleuralia and basalia, in Pheronema 

 giganteum, for instance, two slightly bent hooks, lying in the same transverse axis, occur 

 (PL XLV. fig. 9), and in Polylophus philippinensis four intersecting transverse rays 

 with a gentle curvature (PL LIV. fig. 9). 



In other cases the prominent radial pleuralia, which occur either in bundles or isolated, 

 have the form of simple, terminally pointed diacts. This is the case, e.g., in Acanthascus 

 (PL LVI. fig. 2). In Euplectella suberea the long, slightly bent radial rays of the large 

 jirincipal pentacts of the quadrate lattice-work extend radially beyond the side walls 

 (PL V. figs. 1, 15). In Rossella velata, on the other hand, the external end of the 

 large pentact is that at which the four tangential rays intersect at right angles. 

 All these pentact pleuralia project for nearly equal distances from the surface, and 

 almost touch each other laterally, so that a delicate veil is formed, enveloping the whole 

 Sponge. 



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