274 HYALONEMA (PRIONEMA) SPINOSUM. 



in the gastral surface. The gastral membrane exliibits, in many places, a 

 reticulate structure; in life the mouths of the efferent canals were probably 

 covered with nets. 



The skeleton. The surface is, so far as the dermal and gastral membranes 

 are intact, covered by a dense pinule-fur (Plate 48, fig. 23). The dermal pinules 

 on the outer (lower, convex) side, and the gastral pinules on the inner (upper, 

 concave) side are very much alike. The gastral pinule-fur extends quite down 

 to the bottom of the grooves above referred to. Lateral, paratangentially 

 situated, rays of large (hypodermal and hypogastral) pentactines extend just 

 below the level occupied by the lateral pinule-rays. In the gastral membrane 

 numerous paratangential amphioxes accompany them. The proximal rays of 

 the pentactines point radially inward. Large numbers of stout acanthophores, 

 tetractine to monactine, occur in the protuberance from which, in life, the stalk 

 arose. 



The interior of the sponge is occupied by dense masses of relatively large 

 microhexactines, which evidently form the main support of the whole sponge- 

 body. Besides these spicules, rhabd and hexactine megascleres and amphidiscs 

 occur in the choanosome. 



The dermal pinules (Plate 48, figs. 17-22) are mostly pentactine, rarely 

 hexactine (Plate 48, fig. 20). The distal ray is 100-154 m long, usually 117- 

 138 At, and 3.5-4.5 m thick at the base. It is straight, regularly conic, and not 

 thickened in the middle. The distal end-part of the ray is, for a considerable 

 distance, free from spines. The basal part is also smooth. The remaining parts 

 of it are covered with sparse spines 'directed obliquely upward. The spines are 

 largest on the middle-part of the ray; they are sometimes 11 /x and more long, 

 slender, usually only 1-1.3 /j thick, basally cylindrical, distally conical, and 

 sharp-pointed. The maximum transverse diameter of the distal ray, together 

 with the spines, is 10-17 /x. The lateral rays are straight, sparsely spined in their 

 distal part, and 20-50 ju long, usually 25-40 yu. The proximal ray of the hexac- 

 tine forms is smooth and rarely more than 15 m long. 



The gastral pinules (Plate 48, figs. 23-27) are similar to the dermal and, like 

 these, for the most part pentactine. The distal ray is 100-142 /x long, usually 

 105-135 /x, and 3.5-5.5 n thick at the base. Its maximum transverse diameter, 

 together with the spines, is 11-17 n. The lateral rays are 22-35 n long. 



The hypodermal and hypogastral pentactines (Plate 49, figs. 12-14) have 

 smooth, blunt, conic rays. The lateral rays enclose angles of 90° or a little less 

 with the apical (proximal) ray. The apical ray is straight, or sUghtly curved, 



