160 THE SrONGES. 



elono;ated in the tfing-ential direction. The width of the chambers is 

 approximately the space between adjacent dermal tufts of spicules. In 

 Fig. 5, Plate 19, which represents part of a section radial to the surface, 

 a characteristic subdermal chamber, ^s. c, is shown, which has been cut 

 lengthwise. It connects with a radially extending afferent canal. 



Into the paragastric cavity radial efferent canals open, which lie between 

 the radiating skeletal bundles. These canals, as they are traced toward 

 the surface, branch obliquely. The rounded internal openings of the radial 

 canals measure 250-425 /x in diameter, and are conspicuous in face views of 

 the wall of the paragastric cavity (Plate 19, Fig. 4). Flagellated cham- 

 bers not recognizable. A small commensal annelid is present in the canals 

 in the neighborhood of the surface of the sponge. 



Skeletal Arrangement. The inner part of the sponge wall, next the para- 

 gastric cavity, is strengthened by a framework of spiculo-fibre, consisting 

 of maiu longitudinal fibres 100-300 ju, wide with slenderer connectives 

 (Plate 19, Fig. 4, face view of wall of paragastric cavity ; Plate 25, Fig. 2, 

 longitudinal section through sponge wall). The longitudinal fibres give 

 off branches, which extend radially through the sponge wall (Plate 25, 

 Fig. 2). The radial fibres themselves may branch once or twice before 

 reaching the surface. They or their branches end in tufts of spicules, 

 which project a short distance beyond the surface. Tlie spicules of a fibre 

 "are united by a small amount of colorless spongin. 



Spknles. 1. Strongylc (Plate 19, Fig. 4), curved once or of tener ; 

 smooth ; usually but not always tapering slightly toward the rounded ends; 

 degree of attenuation not always the same for the opposite ends of a spicule. 

 Size, commonly 400-600 /a x 18-20 /a. Smaller sizes down to 280 /n x 12 /a, 

 and larger sizes up to 800 /x x 20 /x are common. Exceptionally stout 

 spicules, for instance, one measuring 714 /a x 26 y^, and exceptionally slender 

 spicules, for instance, one measuring 680 /i, x 14 ju,, are sometimes met with. 



This is the chief spicule in the longitudinal fibres and connectives. It is 

 also present in the radial fibres, though here less abundant than the style. 

 One or two of these spicules may sometimes be found passing between the 

 deeper portions of the radial bundles, as feeble connectives. 



2. Style (Plate 19, Fig. 5), smooth, straight, or somewhat curved at 

 the base, sharp-pointed; sometimes attenuated at the rounded cud. Present 

 in the longitudinal fibres, but not very abundant ; here commonly about 

 600 /x X 22 /x, but larger forms up to 850 /x x 24 /x are found. 



