168 BULLETIN OF THE BUREAU OF FISHERIES. 
The skeleton includes main fibers and connectives. The main fibers ascend in the middle of the 
plate, or lobe, and branch, the branches curving outward in the usual way as radial fibers, which ter- 
minate in the conuli. The ascending and radial fibers are alike, and both are referred to here, in accord- 
ance with the general usage, as main fibers. The radial fibers, in macerated skeletons, are 1.2 to 2 
millimeters apart. 
The main fibers (Pl. LXIV, figs. 43, 44) are characteristically simple, solid fibers, 100 to 200 y thick, 
well filled with mineral particles (sand grains, bits of sponge spicules, and foraminifer shells), all 
comparatively small in size, there being no large sand grains or shells to swell out and distort the fiber. 
The spongin is stratified. In some specimens, but not in others, the outermost ends of the main 
fibers, and the dermal connectives as well, are composed of a much paler spongin than the rest of the 
skeleton, in which the spongin is yellow. This difference is probably associated with some individual 
difference in growth activity. 
Whenever the main fibers appear in any degree fascicular, this is due to one of two causes, as follows: 
(1) Two or even three main fibers may be closely approximated, or a main fiber may branch 
obliquely, the two or three branches continuing to run more or less parallel and close together. Between 
such fibers or such branches, respectively, the connectives are, of course, very short, and the several 
fibers, together with their connectives, constitute a compound fiber such as is characteristic of Stelo- 
spongia. Such compound fibers have a total thickness of about 0.5 millimeter. Thiscondition is found 
here and there in the specimens studied, but is comparatively rare. In the literature on the Stelo- 
spongine when the term “fascicular fiber’’ is used, writers seem usually to have in mind a compound 
fiber of this kind. 
(2) A connective unites with a main fiber by several roots, the middle about transverse to the 
fiber, the upper and lower oblique to the fiber. Thus the roots of a connective attach themselves to a 
considerable extent of the main fiber, forming altogether a sort of triangular plate. If now, as often 
happens, several connectives attach to the same immediate region of a main fiber, but on different 
sides, the main fiber in that region is surrounded by several sets of roots, and thus may appear “‘fascic- 
ular,’’ although in a different sense from that understood under (1). Between any two successive sets 
of roots which meet it the main fiber is, as a rule, obviously simple (fig. 44), but in places the roots 
spread up and down the fiber so far that successive sets meet one another. The main fiber then appears 
not as an independent fiber, but asan axial tract, distinguishable because of its mineral contents, extend- 
ing through an elongated, and close, reticulum. Such a condition is found here and there in the inte- 
tior of the sponge. It is commoner in the outer layer of the body, between the dermal connectives 
and the most superficial of the inner connectives, which usually lie about 1 millimeter below the former. 
In its outermost part, within a conulus, the main fiber often remains simple; but in this region 
the fibers vary considerably, and the variants need to be described. ‘They are as follows: 
(2) The outer end, intraconular portion, of a main fiber is, as said, often simple and so full of mineral 
particles as to show very little spongin. In a typical case it extends 500 » beyond the attaching roots 
of the dermal connectives and ends in a slight enlargement, each dermal connective meeting the fiber 
by several roots. The terminal portion of the fiber may show a perforation or two, due probably to the 
fact that mineral particles here have only recently been surrounded by the spongin of the fiber. 
(2) The roots of the dermal connectives may extend very obliquely upward (and downward) along 
the main fiber. These roots, thus entering a conulus from several sides and extending up toward the 
apex, become interconnected by short fibers, and so may give rise to a very perfect trellis, suspended 
like a tent, as it were, from the uppermost part of the main fiber. This is, perhaps, the commonest 
condition of the intraconular portions of the main fibers in the specimens studied. 
(3) In exceptional cases the main fiber, near its outer end, may divide, the two branches extending 
into the same conulus. They are connected in a close and complex way, the connectives themselves 
being so united as to constitute reticula. Terminally the two branches may end in a common enlarge- 
ment, which maceration shows is a spongin reticulum very full of mineral fragments. In such a case 
the intraconular skeleton is fairly to be classed as ‘‘fascicular’’ in the sense of being a compound, Stelo- 
spongia-like fiber. A variant of this condition is found where two or three main fibers which, in the 
periphery of the sponge, at least, are not branches of a common fiber, converge and enter the same 
conulus, within which they are united by short connectives. 
The most superficial, or dermal, connectives lie in the ectosome just below the dermal surface, 
passing from the outer end of one main fiber to that of another. Except where interconular ridges are 
not developed, they lie in and close to the free edge of the latter. In general they are single, simple 
