SIDONOPS CALIFORNICA. 19 



canals. The radial cortical canals, both the afferent and the efferent, are 

 cylindrical and restricted below by chonal sphincters, which usually protrude 

 slightly into the choanosome. Below these chones subcortical cavities (Plate 

 6, fig. 27b) are mat with. 



The skeleton of the inner parts of the choanosome consists of rather irregu- 

 larly scattered amphioxes (Plate 6, fig. 27e), a few styles, and numerous large 

 oxyasters and sterrasters (Plate 6, fig. 27d). Orthoplagiotriaenes, similar to the 

 subcortical ones, are also occasionally found in the depth of the choanosome. 

 It seems doubtful however whether these are here in their natural postion; 

 they may very likely have been carried into the interior in cutting the sections. 

 The remarkable abundance of sterrasters in the choanosome on the other hand 

 (Plate 5, fig. 27) is without doubt natural. Towards the surface the mega- 

 scleres join to form radial strands which abut more or less vertically on the 

 cortex. These strands (Plate 6, fig. 27c) are composed chiefly of amphioxes 

 and orthoplagiotriaenes. Anatriaenes, anatriaenederivates, and mesoplagio- 

 clades often with reduced cladomes also occur in them, but in much smaller 

 numbers. The cladomes of the orthoplagiotriaenes lie at the limit between the 

 cortex and the choanosome. The cortex is occupied by dense masses of sterr- 

 asters. Small dermal styles are implanted more or less obliquely into its 

 superficial (distal) part. The rounded ends of the styles are situated proxi- 

 mally; their distal, pointed ends protrude freely beyond the surface. These 

 spicules are not very numerous and form tuft-like groups. Those near the 

 afferent pore-sieves and the efferent pores incline towards these apertures and 

 thus form protecting fringes. Groups of such spicules, and single ones, are also 

 met with in the distal part of the choanosome. Numerous minute strongylo- 

 sphaerasters, forming a dense single layer just below the outer surface, are 

 imbedded in the dermal membrane. A few small oxysphaerasters also occur. 

 Besides these numerous other forms of spicules are observed in the spicule- 

 preparations. Most of these probably belong to the incrusting desmacidonid ; 

 there is one, however, an exceedingly minute and slender microamphiox, about 

 which I have my doubts. This may be proper to the sponge and possibly forms 

 dragmes within it. I have, however, not succeeded in finding any of these spic- 

 ules, either singly or in dragmes, m situ in the sections. 



The large choanosomal amphioxes (Plate 5, figs. 11, 12, 27e) are straight or 

 slightly curved, fairly isoactine, and rather abruptly and quite sharply pointed. 

 They are 1.2-2, usually 1.6-1.8 mm. long, and 30-48, usually 38-42 n thick. 



The rare large styles are shorter and thicker than the amphioxes; some 

 attain a transverse diameter of 55 /^ at the rounded end. 



