634 



REVISION OP THE AXINELLIDiE, ii., 



this reticulation ultimately (in the stalk of the sponge) become 

 reduced in size slmost to the point of obliteration. 



In addition to a gradual increase of density, 

 the central axis also undergoes with aire a gradual 

 increase in diameter. This is effected by the con- 

 tinued formation, and addition to it externally, of 

 fresh tracts of megascleres, which later similarly 

 become ensheathed in spongin. In this way, the 

 axial skeleton eventually comes to include within 

 it the lines of spicules which previously constituted 

 the extra-axial skeleton (PI. xxxvi., fig.l). 'J'he 

 extra-axial layer, however, maintains about the 

 same width —viz., about 1 to TS mm.— throughout 

 the whole length of the branches. 



.Sigmata of two sizes are scattered throughout 

 all parts, the smaller in extreme abundance, more 

 especially in the extra-axial layer; the larger ones, 

 which are only moderately abundant, occur also 

 in dragmata. Trichodragmata and singly scat- 

 tered trichites are also moderately abundant, ex- 

 cept in the axial region, where they are rare. 



Meyasd,eres. — These are slightly curved, occa- 

 sionally slightly flexuous styli, almost without 

 exception evenly rounded at the base, and of 

 uniform diameter therefrom to beyond the middle 

 of their length, whence they taper gradually to a 

 sharp point; in very rare cases only, the basal ex- 

 tremity also is more or less pointed, and the 

 spicule may become an anisoxea. They range 

 from 320 to 1525/x in length and up to 18/a in 

 stoutness. Spicules much below 700// in length 

 are relatively scarce. 



Microscleres. — (i.) The larger sigmata are always 

 r L ^ "' more or less contort, though rarely to such a degi-ee 

 V^jsV, > as to appear 3-shaped when seen from the side; the 

 Text-tig. 14."' smaller are usually Q-shaped or but very slightly 



* Siijmaxmella rimmalis. Megascleres and sigmata. 



