. THE SPONGES. 45 



general cylindric.il, or somewhat thicker in the middle region and taper- 

 ing toward the ends, which are enlarged, rounded, and subterminally 

 roughened with microtubercles. The ends may not be swollen and may 

 be smooth. Often, though not always, a trace of the lost rays is retained 

 in the form of a slight annular thickening containing an axial cross. The 

 spicules vary in length from 1 to 4 mm., in thickness from 8 to 12 jm. 

 Lengths of 1.5 to 2.5 mm. are the commoner sizes. Exceptionally the 

 diact is thicker and perceptibly fusiform, tapering evenly from the middle 

 to the rounded smooth points. A typical spicule of this character measures 

 1700 ju, X 24 //,. Bundles of diacts and, less commonly, separate diacts run 

 in all directions through the sponge body (Fig. 10, Plate 5). 



The wall of the stalk is largely occupied by diacts, which run for the 

 most part longitudinally. In the upper part of the stalk these spicules 

 are free. Elsewhere they are connected by abundant synapticula, a con- 

 tinuous framework thus being produced. Scattered diacts protrude radially 

 from the surface of the stalk to a distance of from 1 to 5 mm. The diacts 

 as a class are similar to those of the body, but longer and thicker, many 

 reaching a size 7-8 mm. x 24-32 /x. The extremities may be entirely 

 covered with sharp microtubercles, or the tuberculation may be sub ter- 

 minal. In the lowest part of the stalk some diacts are met with which 

 have smooth, pointed extremities. 



The dermal and gastral pinules (Figs. 7 and 10, Plate 4) are much 

 alike. They are both hexacts in which the proximal and tangential rays 

 are about equal in length and thickness. These rays are pointed and 

 tapering, with very small, sharp microtubercles near the end, elsewhere 

 smooth or with only a few scattered tubercles; about 100 fi x 8-10 /x. In 

 both pinules the distal ray is covered with overlapping upwardly projecting 

 narrow sciiles, which have a greatest length of 16-20 ju,. Near the base 

 the scales degenerate into small prickles, projecting at about right angles 

 to the axis of the ray, and at the extreme base the ray is smooth. The 

 ray ends above in a terminal cone, not in a long point. This in the 

 slenderer spicules is commonly longer than wide, but in the stouter ones 

 is as wide as long and is nearly concealed by the uppermost scales, its 

 tip not infrequently being rounded instead of pointed. 



Except in two specimens the dermal and gastral j^inules differ slightly 

 as regai'ds the length and thickness, and consequently the outline, of the 

 distal ray. In the two specimens referred to, measurements failed to 



