496 A MONOGRAPH OP THE AUSTRALIAN SPONGES, 



As tlie foreign bodies are only found in the centre, the surface 

 of tlie main fibre is smooth. 



The main fibres are generally circular in transverse section, but 

 occasionally they are flattened to form a perforated horny plate. 

 No foreign bodies are found in those portions of the main fibres 

 which are converted into perforated plates. 



The thickness of the main fibre is 0"1 mm. 



The foreign siliceous spicules occasionally attain a length 0-14 mm 



The meshes of the connecting fibres are of the same size near the 

 main fibres as in the intermediate parts of the sponge ; occasionally 

 very regular and measuring 0'23 mm. 



The greater number, 70% of the connecting fibres are of ^i-eat 

 and uniform thickness, measuring 0'04 mm. in diameter. 



There are places in the sponge where all the filjres are thick 

 whereas in others the thinner ones prevail. These measure 

 0"014 in thickness, and where they prevail the net work is much 

 more irregular. 



III. EUSPONGIA IRREGULARIS TENUIS. 



The main fibres of this variety are filled with dense masses of 

 very fine grained sand. Two or more main fibres often extend for 

 some distance parallel and close together. 



Their surface is rough. 



They measure 0-06 mm., in thickness. The knobs are small and 

 close together, The foreign bodies in the main fibres are sand. 

 No foreign spicules have been observed in the main fibres of this 

 variety. The average size of the sand grains is 0*012 mm. 



The main fibres, where they extend in close proximity, are 

 joined to each other by bridges of horn-substance, which often are 

 of great thickness and always free from foreign bodies. 



The connecting fibres form a network which indicates an 

 approach to the difierentiation of tangental primary and radial 

 secundry connecting fibres as it is expressed in the sub-genus 

 Triplicis. This approach however is very slight. Occasionally 



