REPOET ON THE HEXACTINELLIDA. 157 



leugth double that of the principals. Each terminal l)ears at its extremity a small, 

 convex, transverse disc, with recurved marginal teeth (PI. LXIV. fig. 7). In these 

 discohexasters the central node is occasionally much thickened, and provided in the angle 

 between each two principal rays with radial, tubercle-like rounded processes, which may 

 also be drawn out into simple spines. In other cases, one or two of the principals are 

 especially thick, and split up externally into several terminals ; and this modification 

 appears to me to indicate the way in which the numerous rosettes have arisen, which 

 bear eight principal rays arising at approximately equal angles from the central node. 

 The variations in the rays of these rosettes are so numerous that it is impossilile to 

 attempt to describe all the modifications. I shall only note that not unfrequently the 

 central node becomes swollen into a conspicuous sphere, from the surface of which, besides 

 several broad principals, numerous terminals also arise, evidently by the basal splitting 

 of the principals. The splitting of a particular portion may thus increase till the whole 

 principal is divided. 



Under the skin these peculiar discohexasters occur in abundance, but in other regions, 

 and especially in the subgastral trabecular space, peculiarly modified oxyhexasters, 

 oxyhexacts, and remarkable diacts, derived by reduction from the latter, occur abundantly. 

 In numerous oxyhexasters and oxyhexacts, the curved basal portion of the otherwise 

 quite straight, gradually pointed rays, bears a coating of fine spines or barbs, directed 

 obliquely inwards. These barbs are very numerous, and sometimes so long that those 

 of adjacent rays almost unite (PI. LXIV. figs. 8, 9). On these spinous oxyhexacts the 

 rays are sometimes curved, and this not unfrequently takes the form of a spiral twnsting 

 of the two rays on the same axis (PI. LXIV. fig. 10). If it happen, as is by no means 

 unfrequent in these spinous spicules, that the number of rays is reduced, forms result 

 like those represented in PL LXIV. fig. 11, in which a spherical, spinous, central body 

 bears at its two opposite poles two spinous rays, which are twdsted half round in a 

 spiral and then continued in a straight course to end in a simple point, or to be divided 

 into several pointed terminals. But the multiplicity of structure in these apparently 

 reduced forms is so extremely great, that I will not begin to give a detailed account of 

 the multitudinous modifications. 



The dermal skeleton is supported by medium-sized hypodermal oxypentacts, in which 

 the long smooth rays sometimes exhibit a simple curvature, but are, as a rule, quite 

 straioht. In the dermal membrane itself, numerous autodermal diacts occur, in which 

 the rough rays, lying in one axis, end conically or are somewhat rounded oft'. The centre 

 usually exhibits two or four projecting tubercles (PL LXIV. fig. 5), but these are in 

 other cases entirely absent (PL LXIV. fig. 6). There is an isolated occurrence of well- 

 developed tetracts with rays crossed at right angles, and even pentacts with a ray 

 penetrating the parenchyma and resembling that of the diacts. Monacts occur less 

 frequently than in Bathijdorus hacuUfer, but still in tolerable abundance. They may 



