rSAMMASTRA. 39 



Psammastra gigas, u. sp. 



Irregularly spherical massive sponges, which attain a diameter of 200 millira. 

 and are attached by a small base ; with irregular indentations and excrescences 

 on the surface, and one large osculum measuring 20 millim. in diameter on the 

 upper side. Some specimens are contracted towards the base, so as to appear 

 pear-shaped. The surface is uneven and rough ; the colour in spii'it dirty 

 brown. 



The tufts of tetract supporting-spicules, which expand in the surface, allow 

 space for large subdermal cavities between each other ; these cavities appear 

 in the shape of tangential canals, and frequently anastomose to form a per- 

 fect reticulation, which undermines the skin. The tangential canals have an 

 elliptic transverse section and an average width of 0*5 millim. They join to form 

 larger inhalant canals, which extend for some distance in a centripetal direction. 

 These canals are much curved and appear very irregular ; they have an average 

 width of 2 millim., and join below to form large lacunose canals, which pervade 

 the whole of the massive sponge. The latter have an irregularly circular, 

 transverse section, and an average width of 6 millim. They form anastomoses, 

 and in this way a perfect reticulation of inhalant lacunae is produced in the 

 interior of the sponge. From all parts of this inhalant canal-system slender 

 branches originate, which supply the ciliated chambers. Membranous dia- 

 phragms are frequent in the inhalant canals. The exhalant canals are similar 

 to the inhalants, but do not anastomose ; they join to form large branches, 

 which open into an oscular tube passing right through the whole sponge, and 

 measuring 2-5 millim. in diameter. 



Skeleton. — (1) Tetract Megasdera. Abundant in the cortex and scattered 

 throughout the sponge. Angle between the centripetal and the tangential 

 rays ^ 90°; centripetal ray straight, conic, not sharply pointed, 1-7 x 0-03 

 millim. ; tangential rays strongly curved, anchor-like, 0-2 x 0-024 millim. ; 

 curvature and angles very variable. Those tangential rays which are strongly 

 curved are only half as long as the others, or less.— (2) Monact Megasdera. 

 Oxea slightly curved, cylindrical, pretty abruptly pointed, points rounded, 

 1-2 X 0-016 millim., rare in the surface tufts, forming bundles in the interior. 

 —(3) Monad Microsdera. The outer surface contains two varieties of these 

 spicules, which together form a thin cortical layer. All the spicules are more 

 or less vertical to the surface and project slightly beyond it. (A) Spiued stron- 

 gyla 0-048 x 0-005 millim. (B) Slender hair-like rods very thin and smooth, 

 pointed at each end, 0-12 x 0-002 milHm. Both varieties are also found scat- 

 tered throughout the interior of the sponge.— (4) Stellate Microsdera. These 

 are not numerous and very small ; they are found in the interior of the sponge ; 



