44 SPIRETTA. 



fugal monact spicules of the bundles aud the slender tetracts. Colour in spirit 

 grey. No subdermal cavities. Below the thin cortical layer, canals of some size 

 are met with, which form irregular anastomoses and have an average diameter 

 of 0-16 millim. The tissue in the interior is rendered exceptionally loose in 

 consequence of the presence of wide exhalant canals, w^hich join to form 

 oscular tubes 2-3 milim. in diameter ; these are much curved, and terminate 

 in the oscula described above. 



SJcelcton. — The skeleton resembles that of the Tethydse very closely. There 

 is a central sphere of irregularly disposed spicules, 4 millim. in diameter; 

 from which gracefully curved, 0-5 millim. thick, bundles radiate.— (1) 

 Monact Megasdei-a. 8tyli 4 x 0*036 millim., more or less spindle-shaped, and 

 straight or curved like the bundles. These form the bulk of the spicules in 

 the bundles. The pointed ends of the centrifugal spicules project 2 millim. 

 beyond the surface. Some of these spicules may be tylostyli ; such, however, 

 are not frequent. — (2) Tetract Megasdera. Small and insignificant. Angle 

 between the centripetal and tangential rays < 90° (about 60°) ; centripetal 

 ray 1'2 x 0-006 millim., tapering towards a sharp-pointed end, generally slightly 

 curved ; tangential rays anchor-shaped, curved, and sharp-pointed, thick at 

 the base, measuring 0*04 x 0-007 millim. These spicules are more frequent 

 just below the outer surface than in other parts of the sponge and altogether 

 absent in the interior. — (3) Tetract Velar Spicules. Similar in every respect to 

 the supporting-spicules described above. The end of the centripetal ray is 

 planted in the skin. The tangential i\ays of these spicules are in most cases 

 regular, anchored ; sometimes, however, they point outward, the angles between 

 them and the centripetal ray being 140°, so that they appear fork-shaped. 

 Occasionally such spicules are also found in the subcortical layer. In the 

 interstices between the bundles, tetract spicules are found in which the cen- 

 tripetal ray is very long aud thin, measuring 1x0-003 millim.; the angles 

 are variable ; the anchorates predominate, but pitchforks of various shape also 

 occur. — (4) Spiral Microsclera. These are extremely abundant throughout the 

 sponge ; they measure 0-007 X 0-0006 millim. and exhibit 1 or 1| turns. 



Geographical Distribution. — East coast of Australia : Port Denison, Q. 



{Bamsay). 



Familia TETHYOPSILLIDiE. 



Trianiiia with dense masses of radially extending monaxow 

 niegasclera and comparatively few and rudimentary tetracts, with 

 short and irregular tangential rays. 



