298 A MONOGRAPH OF THE AUSTRALIAN SPONGES, 



The I'amifications never lead to the production of a network ; 

 only occasionally we see adjacent fibres coalescing in the middle or 

 also a fibre divided into 2, 3 or more little branches which appear 

 as its roots on one side. 



These fibi-es have a thickness of about 0-02 mm. They are very 

 light in color somewhat like glass and jierfectly transparent With 

 a high power of the microscope a verj slender axiaL thread is made 

 apparent, which swells to little conic granular masses at the points 

 where the axial thread joins the main fibre. 



These fibres are about 0-3 mm. apart from one another. 



Also the cortex of the sponge must be considered as an integral 

 part of the skeleton, and this all the more as we find the main 

 fibres actually coalescing with it. (Plate XX VII., fig. 7.) 



The structure of the cortex is very interesting. Some theoretical 

 conclusions from its peculiarities have been drawn above. The 

 sand granules forming the cortex on the outer exposed surface 

 measvxre on an average O'l mm. in diameter, and what is very 

 remarkable, are all of the same size. 



The sand granules in the interior of the main fibres are identical 

 in size and shape with those in the outer exposed surface, and 

 without a doubt derived therefrom. (1) The.se sand granules are 

 held together by a cement of spongiolin, and the armor they 

 form has a thickness of about 0-35 mm. — a thickness very similar 

 to that of the main fibres. 



The canals leading into the vestibule-lacunfe are short and 

 circulai', cylindrical perforations of the external lamella. (Plate 

 XXVII., figs. 4, 7 ; 0.) 



In the wall of these short tubes we find a very difierent kind of 

 cortex. It is a transition from the cortex without to the cortex 

 within. Near the outer surface (compare the figui-e) the sand 

 grains are as large as in the outer cortex, but the layer they form 

 is only about half as thick. 



As we proceeded downward the size of the sand grains and also 

 the thickness of the sand layer decrease rapidly, until finally we 



(l)Tliis is in accordance with the views expressed by Carter and others. 



