REPORT ON THE HEXACTINELLIDA. 141 



section, so that the whole spicule comes sometimes to look like a many-rayed star. The 

 principal i"ay is usually divided only into two, but not unfrequently three diverging 

 terminals are thus produced, and less frequently more tliau three. Besides these 

 oxyhexasters, there is a very abundant occurrence of various discohexaster forms, which 

 are in part somewhat smaller than the oxyhexasters just described, but in part larger. 

 These also exhibit short simple principal rays, and comparatively long divergent 

 terminals, with small four- to six-toothed, transverse and somewhat recurved terminal 

 plates. The number of terminal rays varies very considerably in the smaller 

 forms ; there are usually three to six or even more on each principal ray (PL LV. figs. 

 6, 15). 



In the larger discohexasters, which are also rather divergent in form, the simple 

 cylindrical principals are somewhat longer, and divide into from three to six S-shaped thin 

 terminals, which are grouped together in a slender perianth-like bundle (PL LV. fig. 14). 

 Carter has already noted this peculiar form of rosette as characteristic of the species. 

 Of quite isolated and exceptional occurrence in the parenchyma is the small simple 

 discohexact, figured in PL LV. fig. 8. It may possibly not belong to this sponge, but 

 have originated as an intrusion from without. 



The dermal skeleton includes, in the first place, medium-sized hypodermal 

 oxypentacts, in which the rays are for the most part smooth, and only roughened at the 

 conically pointed ends. The four cruciate tangential rays are rarely disposed exactly in 

 the dermal membrane, but are usually somewhat below it, or pushed out beyond, 

 though in both cases parallel. In some cases the four tangential rays do not intersect at 

 right angles, but are all pushed together to one side, as we shall afterwards have to note 

 in regard to the pleural prostalia. In much closer connection with the dermal membrane 

 are the small, finely spinose, autodermal pentacts. In these the four cylindrical tangential 

 rays, which are tolerably straight, and at most gently incurved, with rounded or somewhat 

 conically pointed ends, form a rectangular meshwork within the dermal membrane. The 

 proximal radial ray, which has approximately equal length and similar characters, 

 projects into the subdermal space (PL LV. figs. 2, 3). The undeveloped sixth distal 

 radial ray is almost always represented by a rounded tubercle or knob. In rare cases, 

 especially near the base, I found, between the pentacts, isolated cruciate tetracts of 

 similar form and equal size (PL LV. fig. 4). 



The gastral skeleton, which lines the inner surface of the gastral cavity, essentially 

 resembles the dermal, Ijut dificrs in this, that the finely spinous gastralia, whose 

 tangential rays extend within the gastral membrane, are not pentacts, but weU-developed 

 hexacts (PL LV. fig. 5) in which the internal radial ray projects into the gastral ca^dty, 

 and gives the internal surface that peculiar velvety appearance which we noted above. 



The radial tufts of pleural prostalia, which project from the papilke over the whole 

 external surface of the body, consist of four to eight spicules which project for 1 to 2 cm. 



