HYALONEMA (PRIOxNEMA) AZUERONE. 267 



paratangentially in the superficial membranes and occupy, singly or in bundles of 

 two or three, the axes of the threads of the nets covering the afferent pores. 

 Similar rhabds traverse the choanosome, singly or in bundles, in various directions. 

 Most of these rhabds are centrotyle isoactine amphioxes. In some, one actine 

 is reduced in length and terminally thickened; these resemble tylostyles. Pentac- 

 tine megascleres occur in the superficial parts of the lamellae. In the interior 

 a few hexactine forms are found. Very numerous microhexactines and a few 

 pentactine and diactine-derivates of these spicules are also found in the interior. 

 Seven kinds of amphidiscs occur in this sponge : — not very numerous macramphi- 

 discs with serrated anchor-teeth; very rare large mesamphidiscs with smooth 

 teeth; very numerous medium mesamphidiscs, which, in places (Plate 58, fig. 2), 

 form quite dense masses ; a few similar small mesamphidiscs ; numerous slender- 

 shafted regular micramphidiscs ; and two kinds of micramphidiscs, a larger and a 

 smaller, which are stout-shafted, and generally more or less irregular. 



The superficial {dermal and gastral) pinules (Plate 58, figs. 3a, 10, 14, 15, 17, 

 18, 20-22) observed were all pentactine. The distal ray is straight, 190-390 n 

 long, and 5-9 n thick at the base. It ends with a very slender sharp-pointed 

 terminal cone, and the whole of it, with the exception of its proximal and distal 

 end-parts, is beset with spines. These spines are numerous, rather crowded 

 and longest in the middle-part of the ray ; they decrease in size both proximally 

 and distally. The lowest arise nearly vertically; distally they become more 

 and more inclined towards the tip of the ray. The longest spines of the middle- 

 part of the ray usually enclose angles considerably less than 45° with the axis of 

 the ray. These spines are conic, sharp-pointed, attain 25 m in length, 3 n in 

 thickness, and are slightly cur\-ed, concave towards the tip of the ray. They 

 are either simple, or bear one or two outwardly directed branch-spines, which 

 sometimes reach a very considerable size (Plate 58, fig. 18). The maximum 

 thickness of the distal ray, together with the spines, is 22-36 n. The basal half 

 of the lateral rays (Plate 58, figs. 14, 15) is nearly cylindrical and smooth, the 

 distal half conic and provided with somewhat sparse, quite large, broad, and low 

 spines. The end is blunt. The lateral rays are 25-55 n long. They appear to 

 be longer in the gastral than in the dermal pinules; in the former they are usually 

 about 40 (U long, in the latter about 30 m- 



The canalar pinules (Plate 58, figs, lb, 16, 19) are on the whole similar to 

 the superficial ones but have more slender rays, a shorter distal ray, and fewer 

 and smaller spines on the latter. It is also to be noted that they are not all 

 pentactines, a few hexactine forms occurring among them. The measurements 



