SPONGES. 73 



below. The apparent total absence of vents is a remarkable feature. Only in the 

 smallest specimen have I seen what looks like a small natural vent, and that is on 

 the flattened (presumably lower) surface. Doubtless the exhalant apertures are 

 completely concealed by contraction in preserved specimens. Owing to the enor- 

 mously strong development of the dense radiating skeleton, and the manner in which 

 the cladi of the triaeiies come close up to the surface (giving it its granular character), 

 I have found it impossible to cut a tangential or surface section in the ordinary way 

 in order to look for pores in the dermal membrane. On attempting to cut such a 

 section the sponge splits up radially along the lines of the gigantic spicules. 



The skeleton is remarkably strongly developed, the shafts of the huge oxea and 

 trieenes being arranged in dense bundles, which radiate from a central " nucleus :: and 

 leave only narrow interspaces between them ; while the expanded cladi of the 

 dichotrisenes form a thin but dense dermal crust, in which innumera'ble very minute 

 asters also occur. 



Spicules. -(1.) Dichotrisenes ; shaft about 8'0 millims. long by 0'074 millim. thick 

 a short way below the cladome, tapering very gradually to a narrow, long-drawn-out 

 but rounded point ; cladi short, stout, once-forked, expanded almost at right angles 

 to the shaft ; chord about 0'46 millim. 



(2.) Protrisenes ; with long and very slender shafts and slender cladi about 

 0'05 millim. long, commonly projecting from the general surface. Much larger 

 protrisenes, with comparatively stout shafts and short stout cladi, occur in the 

 anchoring tufts on the lower surface of the adult sponge. 



(3.) Anatrisenes ; with long slender shafts, and" rather stout cladi up to about 

 O'l millim. long. 



(4.) Oxea; associated in bundles with the shafts of the dichotriaenes ; fusiform, 

 gradually and fairly sharply pointed at each end; size about 8'0 millims. by 0'073 

 millim. 



(5.) Asters; very numerous at and near the surface of the sponge, with small 

 centrum and numerous slender conical rays of equal length ; total diameter about 

 O'Ol millim. Such spicules are very rare in the deeper parts of the sponge, and 

 those which I have seen do not differ in any important respect from the asters of the 

 surface. 



The smallest specimen possesses in one part, beneath the surface, in addition to 

 the asters, numerous minute refractive globules, which look as if they might be 

 siliceous. 



Stained sections show that there is a very thick gelatinous (collenchymatous) 

 ectosome, excavated by numerous large, irregular cavities, from which occasionally 

 wide canals, provided with numerous transverse diaphragms, lead vertically inwards 

 through the choanosome. Small dermal pores are scattered between the cladi of the 

 dichotrisenes. The choanosome is not sharply differentiated from the ectosome, but 

 is finely granular. The flagellate chambers are oval and measure up to about 



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