SIDONOPS BICOLOR. 



49 



stout and slender ones, can be distinguished. Tlie former occur chiefly in tlie 

 axial parts of the bundles, while the latter predominate in their superficial parts. 

 In the distal portions of the spicule-bundles plagiotriaenes also occur. The 

 cladomes of most of these plagiotriaenes lie at or just above the limit between 

 choanosome and cortex, the clades being often quite enveloped by sterrasters; 

 their rhabdomes extend radially inward. The radial spicule-bundles abutting 

 on the afferent areas do not terminate at the cortex, but penetrate it (Plate 9, 

 fig. 16) and protrude beyond it, thus forming the fur. The fur consists chiefly 

 of amphioxes but plagiotriaenes also take part in its formation. In the spicule- 

 fur of a specimen from Station 2958 I found a good many plagiotriaenes, with 

 rhabdomes implanted in the cortex, and free cladomes lying a considerable 

 distance above the surface of the sponge. Where these spicules arise from it, the 

 surface is often raised conulus-fashion. In a specimen from Station 4551 I have 

 found a few styles and in the spicule-preparations of specimens from Stations 

 2958 and 3168 two small protriaenes. The latter are probably foreign spicules. 

 Very small and slender rhabds are often found imbedded in the superficial part 

 of the cortex. I think it highly probable that these belong to the symbiotic 

 monaxonid sponges which incrust parts of the surface. 



The microscleres are strongylosphaerasters, smaller oxysphaerasters with 

 numerous rays, larger oxyasters with fewer rays, and sterrasters. The strongylo- 

 sphaerasters form a dense layer on the outer surface (Plate 10, fig. 15) and are 

 absent in the interior. The oxysphaerasters occur chiefly in the walls of the 

 cortical canals and are also met with in the region of the subcortical cavities. 

 The oxyasters are restricted to the choanosome, in the walls of the canals of 

 which they are very numerous. The sterrasters occupy the whole or nearly the 

 whole of the thickness of the cortex (Plate 9, figs. 15-16) in dense masses. In 

 most of the specimens the choanosome is free from sterrasters. In some how- 

 ever, particularly in the specimens from Stations 3168 and 4420, considerable 

 numbers of sterrasters, chiefly young ones, were found in it. 



The stout amphioxes (Plate 9, figs. 9-11) are curved, isoactine, or slightly 

 anisoactine, attenuated towards the rather blunt ends, gradually in the central 

 parts and rather abruptly in the distal parts. They are 2.3-5.6 mm. long and 

 35-105 n thick. An inverse proportion between length and thickness is indi- 

 cated. Those of the specimens from Station 4551 attain a greater maximum 

 thickness than those from the other stations. 



