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



the latter genus is absent, and the margin curves over and downward on the inside to a 

 considerable distance at least, how far is yet unknown." Moreover, he erects a species 

 with the name CyatJiophycus suhsphgericus, and adds that " each species preserves the 

 rounded rim of the circular aperture at the summit." 



Character of the Genus. — ^The more or less regularly disposed circular parietal gaps 

 are surrounded by a soft membrane provided with circular muscle fibres. The tubular 

 or slightly swollen body runs out into a tuft of siliceous fibres at the narrowed inferior 

 extremity, while the upper, transversely truncated end is closed by a flat or outwardly 

 convex sieve, and bounded by a thickened border, which is continued into a wreath of 

 spicules, or into a cuff-like fringe. The larger parenchyma spicules have in the later age 

 a tendency to fuse together, and in adult specimens may unite to form a continuous 

 framework. 



The groundwork of the whole skeleton is a system of intersecting longitudinal and 

 circular siliceous strands, which consist of the prolonged rays of strong hexacts, pentacts, 

 or tetracts, and of the apposed thread-like, thin, comital tetracts, triacts, and diacts. The 

 intersections of the fibrous strands form quadrate meshes. The rosettes scattered in the 

 parenchyma are oxyhexasters. The terminal plates of the typical protuberant floricomes 

 have from six to eight teeth. 



1. Euplectella aspergillum, R. Owen (Pis. I. -IV.). 



In general form Euplectella aspergillum resembles a thin-walled tube of approxi- 

 mately circular section ; the somewhat narrowed lower end is continued downwards into 

 a bundle of longitudinally directed or slightly diverging siliceous fibres, while the 

 superior transversely truncated extremity is closed by a watch-glass-shaped, outwardly 

 convex sieve-plate. In its upper two-thirds the tube is straight or only slightly bent, 

 while the inferior third usually exhibits a more marked, often almost semicircular 

 curvature (PL I. figs. 1, 2). If there are two curves, they usually lie in the same jilane, 

 but run in opposite directions, so that a form resembling a slightly bent, S-shaped cornu- 

 copia results. The length of the full-grown specimens, exclusive of the root-tuft, which 

 measures from 4 to 8 cm., is on an average 24 cm.; the breadth, in the upper portion, 

 varies from 4 to 5 cm.; the lower portion, just above the basal tuft, measures from 2 '5 to 

 3 cm. The dimensions of the individual specimens vary, however, very considerably. 



The whole wall of the tube is perforated by numei'ous parietal gaps, circular apertures 

 of tolerably regular form, and varying from 1 to 2 mm. in diameter. These gaps lie at 

 the bottom of furrow-like grooves, and each gajJ is surrounded by a stretched, soft, iris- 

 like, circular membrane, provided with a circular muscle. The arrangement of the 

 parietal pores is such, that two intei-secting systems of parallel right and left-handed 

 spirals, ascending at an angle of 45°, may be readily recognised. This regularity. 



