ACANTHASCÜS CACTUS. 149 



are however never numerously present. Usually the spicular center 

 is externally provided with a gentle annular swelling ; more rarely, 

 with two opposite protuberances. 



When the sponge is torn from the rock to which it was 

 attached, the basidictyonal plate is left adhering to the latter. 

 The plate can easily be separated out by boiling in acid. It is 

 found to be in places quite thin, being represented by an uneven 

 and small-meshed siliceous reticulum (the limiting layer of the 

 basidictyonal plate, see p. 134), the beams of which may at 

 intervals show^ spicular axial filaments in the form of a plane 

 cross (as in PI. XXII., fig. 17). At other places there exists, 

 besides the same limitin s; or attachment laver and in direct union 

 with it, a somewhat wider-meshed sponge-work (PL XII., fig. 37) 

 which increases the thickness of the basidictyonal plate to half 

 a millimeter or more. The relalively thick beams of the sponge- 

 work are rough-surfaced all over on account of the presence of 

 numerous conical microtubercles. The manner of their arrange- 

 ment is apparently irregular, but by exact observations it is not 

 difficult to make out that the foundation to the structure is given 

 by certain hexactins, the basidictyonalia, which are directly as 

 well as synapticularly fused together. Isolated instances also occur 

 of diactins or of parts of diactins — apparently belonging to the 

 parenchymalia — being incorporated in the beams. Observed in 

 glycerine, such a diactin is seen to be of smooth-contour and 

 distinctly traversed by the axial canal but entirely enveloped by 

 an irregular and synapticula-forming secondary deposit of siliceous 

 matter. But by far the greater part of the parenchymalia in the 

 region have their lower end simply loosely inserted into the 

 interspaces of the basidictyonal framework. 



