384 HYALONEMA (SKIANEMA) UMBRACULUM. 



2 mm. thick. It has obviously been strongly compressed during or after capture, 

 and I do not think that the living sponge, of which it once formed a part, was 

 at all lamellar. The other specimen (fragment) is similar but much smaller. 

 The colour in spirit is light brown. 



The skeleton is composed of superficial pinules, hypodermal and hypogastral 

 pentactines, hexactine megascleres, choanosomal and superficial rhabds, acan- 

 thophores, axial amphioxes forming an upper continuation of the stalk in the 

 sponge-body, stalk-spicules proper, microhexactines, diactine microhexactine- 

 derivates, and amphidiscs. The hexactine megascleres are exceedingly scarce. 

 The superficial and choanosomal rhabds are for the most part centrotyle am- 

 phioxes, but diactine tylostyles also occur among them. The microhexactines 

 are abundant, their diactine-derivates very rare. The amphidiscs are of foiii' 

 kinds: — ^ large and small macramphidiscs, and large and small micramphidiscs. 

 The small micramphidiscs are remarkably scarce. 



The dermal and gastral superficial pinules (Plate 103, figs. 9-13) appear 

 to be quite similar. All the pinules observed were pentactine. The distal ray 

 is straight, 178-290 ^ long, most frequently about 240 yu, and 4-11 m thick at 

 the base. It ends with a rather long and slender spineless terminal cone, and 

 its basal part is also free from spines. The middle-part of the distal ray is, for 

 about two thirds of its length, covered with straight or slightly curved spines, 

 which are strongly inclined towards its tip. The spines situated half way up 

 the ray are the largest. The maximum thickness of the distal ray, together 

 with the spines, is 27-40 ix. The lateral rays are cylindroconical, blunt, spiny, 

 and 28-48 m long. 



The rays of the hypodermal and hypogastral pentactines are cylindroconical 

 and very blunt. In the intact pentactines observed the proximal ray is 370- 

 480 M long, and 14-18 n thick at the base, while the lateral rays attain a length 

 of 120-240 fi] however, judging from the fragments of larger ones found in the 

 preparations, the rays of these pentactines must frequently attain a much larger 

 size. The largest lateral rays of fragmentary pentactines observed attain 1 mm. 

 in length and 70 m in thickness at the base. These very large fragments may, 

 however, be parts of foreign spicules. 



One of the very rare hexactine megascleres measured is 480 m in maximum 

 diameter, and has tei-minally rounded, cylindroconical rays 12 fi thick at the 

 base. One of the rays of this spicule is considerably longer than the other four. 



The choanosomal and superficial amphioxes are centrotyle, 0.5-2.5 mm. long, 

 usually 1-1.5 mm., and 11-26 m thick near the centre. The central tyle is 14- 



