REPOET ON THE HEXACTINELLIDA. 75 



rays are never so much prolonged. Hexacts seldom occur among the parenchymal 

 spicules, and like the somewhat more frequent pentacts are irregularly developed. The 

 numerous tetracts which are present, exhibit, indeed, for the most part, rectangularly 

 intersecting axes, but these seldom lie entirely in a plane, being for the most part 

 uniformly bent towards the surface ; it may be that the two rays belonging to one axis 

 run out in the same direction and straight, while the two others are somewhat bent, or 

 are disposed at an obtuse angle to one another. Sometimes tetracts occur with one of 

 the rays disposed at right angles to the surface of the three others. The majority of 

 these parenchymalia have only two or three rays. Of the triacts the two rays belonging 

 to the same axis are usually bent in a slight curve, from about the middle of the convex 

 or concave side of which the third straight ray sjarings (PI. V. fig. 16). The diacts are 

 sometimes straight, sometimes slightly bent, pointed at both ends, or more or less 

 rounded. They exhibit in the middle of their axial cross, four, two, or one tubercle, 

 and are frequently roughened, not only near their ends, but throughout. 



Peculiar small, straight, rough diacts, with truncated or rounded extremities, and 

 with four tubercles projecting markedly from the axial cross and often exhibiting central 

 terminal points (PI. V. fig. 7), occur abundantly in the thin circular membrane which 

 surrounds each of the parietal gaps. As a rule, the two rays are equally long, but forms 

 also occur with two long unequal rays, and others which by the very great shortening of 

 one ray have become monacts. 



Both in the outer and inner trabecular framework rosettes occur, scattered quite 

 irregularly in the form of oxyhexasters, whose short principal rays bear usually three 

 (PL V. fig. 2), seldom four, straight, long, more or less stout terminal rays. The rosettes 

 represented by Wyville Thomson on PI. V. figs. 3, 8, I have only discovered in those 

 greatly damaged fragments which were much mixed with the spicules of other 

 Hexactinellida, and which were collected ofi" the coast of Brazil (Station 124). These 

 latter rosettes probably do not belong to Euplectella suberea, but appear to me to have 

 originated in other Hexactinellida, and to have become accidentally embedded in these 

 specimens. At least in sections of Euplectella suberea (collected to the west of Gibraltar), 

 I have never been able to find such rosettes in situ. 



The dermal skeleton consists of a layer of dagger-like delicate h}q)odermal hexacts 

 with rough conically pointed extremities, which have their tangential rays so apposed to 

 one another under the bounding membrane that a rectangular meshwork is formed, while 

 the proximal ray penetrates deeply into the parenchyma, and the distal ray, extending 

 to the very tip of every minute tubercle of the skin, bears the somewhat freely projecting 

 floricome (PI. VI. fig. 3). 



It is noteworthy that on the top of those flat archings which project outwards 

 between every four parietal pores, the much prolonged distal ray of a specially large 

 hexact usually projects at right angles to the surface, and instead of carrying a floricome 



