26 ANATOMY AND PHYSIOLOGY 



In Tethea Norvegica and Int/alli, Bowerbank, MS., and 

 in T. lyncuriinn, Johnston, the same protection is attained 

 in a different manner. Instead of the spicula of the 

 skeleton fasciculi gradually converging towards a point, 

 they diverge considerably as they approach the surface, so 

 as to present an infinite number of minute and nearly 

 equidistant points, and in addition to these the dermal 

 membrane and the coriaceous coat of the sponge is supplied 

 with an infinite number of closely packed stellate spicula. 



In some species of the genus Geodia the system of 

 external defences is still more complex. Thus in G. 

 McAndrewii and G. Barretti the defences are double, one 

 system consisting of a continuation of the great radial 

 fasciculi of the skeleton as a protection against the assaults 

 of the larger and more powerful assailants ; and then of a 

 secondary series consisting of an infinite number of minute 

 acerate spicula, based immediately beneath the dermal 

 membrane and projecting to a slight extent beyond its 

 external surface, effectually protecting it and the porous 

 system of the sponge from the attacks of its minute and 

 more insidious enemies, 



Similar modes of external defences exist in various 

 species of Packymatisma and Ecionemia, but no two 

 .species appear to agree precisely in these respects. 



In the genera Microdona and Hymeraphia, Bowerbank, 

 differing widely in the structure of their skeletons from any 

 of the sponges hitherto described, and frequently not 

 exceeding in thickness the substance of a stout sheet of 

 paper or a thin card, the same principles of defence are 

 carried out, although their structure is widely different 

 from each other. In the first genus, the skeleton of which 

 is formed of short pedestals of keratode combined with 

 spicula, each of the pedestals, which reach nearly to the 

 surface of the sponge, is terminated with a radiating 

 cluster of long curved and acutely-pointed spicula, the 

 apices of which pass through the dermal membrane in 

 every direction, and thus form a most effectual series of 

 external defences, while their shafts beneath serve as the 

 framework of the mtermarginal cavities of the sponge 

 (Figs. 368, Plate XXXIII, and 369, Plate XXXIV). In 



