SPONGES. HALLMANN. 



183 



■ciable extent, may also invade the fibres ; and, thirdly, there 

 are the special dermal styli (much smaller in size than either of 

 the preceding), which are arranged at the surface, but which, 

 like the others, are also to be found in the ground tissues. In 

 the dermal skeleton the auxiliary styli (proper) are disposed 

 horizontally in broad lines (in which they lie roughly parallel) 

 forming a reticulation, the mesh of which averages about 

 160 ji in diameter. The special dermal styli stand perpendicu- 

 larly upon the sides of this meshwork, echinating them, as it 

 were. The special dermal styli usually exhibit a small area 

 of spination on the basal extremity ; a similar, though more 

 minute, spination is also of occasional to frequent occurrence 

 on the auxiliary spicules ; but it is always of more or less rare 

 occurrence in the case of the principal styli. The drawings in 

 the text-figure have been arranged so as to show the very 

 perfect gradation of one spicule form into another. 



Megascleres. — 



(i.) The principal styli are usually slightly curved. Their 

 maximum size varies in different specimens from 280 

 x 10.5 to 300 x II |i. (In the divergent specimen of 

 T. ruhens var. lamella, spicules larger than 220 x 8 )t 

 are exceedingly rare. A single spicule 280 x 10.5 ]i 

 was observed, and several others of intermediate 

 length, in some of which the diameter exceeded 9 }i.) 



(ii.) The acanthostyles are rather minutely spined ; the 

 spines are recurved and either scattered closely over 

 the whole length of the spicule or are more or less 

 reduced in number on the region extending from the 

 neighbourhood of the very densely spined head 

 towards the middle of their length. They are conical 

 tapering spicules which rarely exceed a size of 95 x 

 7 ji, though individuals up to 105 x 7.5 yi have been 

 observed. 



(iii.) The auxiliary styli (proper) are straight. Those in 

 those in the ground substance vary greatly in dia- 

 meter, owing to the admixture of immature indivi- 

 duals. The latter are frequently tylostyli, but, as their 

 development proceeds, their basal enlargement is 

 gradually effaced, though traces of it are often re- 

 tained by full-grown spicules. Those of the dermal 

 skeleton are very uniform in stoutness, and being 

 unmixed with principal styli allow their size, as dis- 

 tinguished from that of the latter, to be readily 

 determined. They may attain to the length of the 

 principal styli, but are usually slightly shorter. 

 Maximum size, 270-295 x 8 jt. (In the abnormal 

 .specimen they attain to 280 x 7.5 \i.) 



