134 EUSPONGIA. 



covers the subdermal cavities is on an average 0*06 millim. thick. The latter 

 are formed by the repeated anastomosis of tangentially extending canals of 

 circular transverse section, which undermine the skin throughout the entire 

 surface. These canals vary very much in size ; they measure from 0"25-l 

 millim. in diameter, and extend in the digitate and lobose processes mainly in a 

 longitudinal direction. Very few and distant larger inhalant canals extend 

 from these downward into the interior of the sponge, where they ramify very 

 much and give rise to numerous cylindrical canals, which extend mainly in a 

 longitudinal direction. These measure 0-05 millim. in diameter, and are 

 surrounded by the ciliated chambers. The latter open into wider exhalant, 

 likewise longitudinally disposed canals, which measure on an average 0*2 millim. 

 in diameter. The latter coalesce to form wide, irregularly disposed tubes 

 which lead, extending in a tangential direction, into the extensive lacunse of 

 the exhalant system. 



It has been mentioned above that the oscula are usually surrounded by 

 aconulous zones, which are destitute of horny fibres and represented in the 

 skeleton by irregular grooves and depressions. These askeletous parts are 

 occupied by an exceedingly lacunose tissue, with few chambers and very wide 

 exhalant canals. 



The structure of the sheleton of the different varieties agrees in the following 

 points : — The main fibres are cored with foreign bodies, and are on an average 

 1 millim. apart. They are not much curved, extend radially from the base 

 of the sponge upward and outward, and are branched in a penicillate manner. 

 The connecting-fibres form a very regular network, when they are of uniform 

 thickness, but when the thickness of the connecting-fibres varies the meshes 

 become irregular. The network is more dense in the vicinity of the main fibres 

 than elsewhere. The connecting-fibres are always free from foreign bodies. 



Euspongia irregularis, var. silicata, Lendenfeld. 



Eusjpongia irregularis, var. silicata, E. v. Lendenfeld, " A Monograph of the 

 Australian Sponges. — Part VI. The Genus Euspongia" Proceedings of the 

 Linnean Society of New South Wales, vol. x. p. 495 (1885). 



The main fibres contain abundant foreign bodies in their axis ; these are 

 chiefly siliceous spicules of other sponges, forming 90 per cent, of all the 

 foreign bodies ; the other 10 per cent, are Poraminifera — shells, &c., but there 

 is hardly any sand. As the foreign bodies are only found in the axis, the 

 surface of the main fibres is smooth. The main fibres generally have a cir- 

 cular transverse section, but occasionally they are flattened and form perforated 

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

 which are converted into perforated plates. The thickness of the main fibres is 



