SILICIOUS AND HORNY SPONGES WILSON. 307 



Surface glabrous. Cortical layer whitish brown in color, interior- 

 darker. Interior shows many canals, the larger 5-8 mm. in diameter 

 and abundant small ones I-IV2 mm. diameter. 



The cortex is about 1 mm. thick, occupied almost exclusively by 

 the sterrasters. The ectochrote is represented only by a very thin 

 dermal membrane, and the innermost, fibrous, layer of the cortex is 

 also quite thin. 



The chones are relatively simple structures in this species. The 

 uniporal incurrent chone-canals (pi. 45, fig. 16, i. ch. c.) are wide, 

 straight, subcylindrical tubes that pass radially through the whole 

 cortex, each opening below into a larger subcortical canal (s. p.). 

 The aperture into the latter is much narrower than the chone-canal 

 in general and is guarded by a fibrous sphincter (sph.). Above, 

 the chone-canal is closed in by an area of sterraster-free dermal 

 membrane, full of microrhabds, perforated by the pore (p.). A 

 representative incurrent chone gives the following measurements: 

 Pore, 350 \l in diameter; rim of sterraster-free dermal membrane 

 surroimding the pore, 85-180 jjl wide ; diameter of chone-canal, 850 p. ; 

 diameter of aperture into subcortical canal, 525 y.. 



The oscula are the apertures of uniporal excurrent chone-canals, 

 essentially similar to the incurrent canals although somewhat wider. 

 The sphincter at the inner end of the excurrent chone, guarding the 

 aperture into the subcortical canal, is possibly narrower on the 

 average than in the case of the incurrent chone. A representative 

 excurrent chone gives the following measurements: Osculum, 1 

 mm. in diameter; rim of sterraster-free membrane surrounding it, 

 100 \k wide; diameter of chone-canal, 1,450 [/,; diameter of aperture 

 connecting with subcortical canal, 1,150 jju The sphincter of this 

 chone is therefore 150 [t. wide. 



There are some slender skeletal tracts, composed of monaxon 

 megascleres, in the interior. Some of these are continuous with the 

 radial bundles of the periphery of the sponge. The radial bundles, 

 each consisting of a triaene with numerous monaxon megascleres, 

 pass to the cortex which they do not enter. They are abundant, 

 often 600-1,200 n apart, as seen in sections. 



Spicules. — 1. Orthotriaene (pi. 46, fig. 1). Rhabdome, straight or 

 nearly so, in a radial bundle. Cladome tangential, just beneath the 

 cortex. The clads may be nearly straight, but are usually somewhat 

 curved or bent like a horn, often conspicuously so ; the end of a curved 

 clad always pointing inward. One of the clads is occasionally dichoto- 

 mous. Rhabdome and clads not far from the same size, y 2 mm. 

 long, 50 \l thick at the base. The rhabdome is sometimes a little 

 longer than the clad, and the clads of a spicule may differ some- 

 what in length. 



