26 HOLASCUS EDWARDSII. 



of the anatriaenes of Thenea valdiviae ' and other tetraxonid sponges. In these 

 the branchlets, however, appear to be rudiments of primary axial threads and 

 morphologically equivalent to the axial threads of the rhabdome and the fully 

 developed clades. Here in Holascus edwardsii they can only be considered as 

 (secondary) axial thread-branches equivalent to the axial thread-branches in 

 the end-clades of dichotriaenes. 



The large choanosomal pentactines (Plate 18, fig. 22a; Plate 20, figs. 1, 2) 

 have straight or slightly curved conic rays, which are 75-145 m thick at the 

 base. The lateral (paratangential) rays are long and form the edges of a low 

 quadrangular pyramid, from the apex of which the short apical (distal) ray 

 arises. The lateral rays, which extend longitudinally, are 13-19 ^ long, the lat- 

 eral rays, which extend transversely, 7-11 mm. long. The apical ray has a 

 length of about 1.5 mm. The terminal parts of the rays bear scattered, small, 

 broad, and blunt spines. The other parts of the rays are smooth. 



In the distal part of the lateral rays of these pentactines the axial thread is 

 thickened at frequent intervals. These thickenings are, on the whole, conic 

 and consist of verticils of short, rod-like, axial thread-branches diverging only 

 slightly from the axis of the ray, extending backward centripetally and a little 

 outward. The slightly granular substance, of which these rods consist, is 

 apparently the same as the substance composing the axial thread. Sometimes 

 a small cap, with the convex side turned towards the distal end of the ray, is 

 found within the conic rod-verticil, just below its apex. These axially situated 

 caps consist of a substance with a refractive index very different from that of 

 the substance of the axial thread and the silica-layers of the spicule, and are 

 consequently, in spite of their small size, very conspicuous. They look as if 

 they were portions of tissue rich in water, entrapped by the growing spicule 

 (axial thread). The conic verticils of rods (axial thread-branches) and these 

 caps indicate that the growth of the lateral rays of the pentactines is inter- 

 mittent; the rod-verticils and caps marking the positions of the ray-tips at the 

 times of suspension of longitudinal growth. Each node of the ray between two 

 adjacent thickenings of the axial thread is doubtlessly produced by the unin- 

 terrupted work of a spicule-builder or a set of spicule-builders. The secession 

 of work by the cell or cells causes the interruption of growth. After a time 

 the same spicule-builder or same set of spicule-builders or a fresh one or fresh 

 set recommences or commences work, whereupon the growth again goes on. 



The comitals (Plate 18, figs. 15, 16, 22b, 23) which hold the large pentactines 



' B. V. Lendenfeld. Die Tetraxonia. Ergeb. Deutsch. tiefsee-exped., 1907, 11, p. 200 ff. 



