OF THE SPONGIAD.E. 19 



exists in the beautiful harrow-shaped tissue of the dermal 

 structures of the sponge Farrea occa, Bowerbank, support- 

 ing the fine specimen of Ewplectella in the possession of 

 my friend Dr. A. Farre, and described by Prof. Owen in 

 the 'Transactions of the Linnean Society,' vol. xxii, p. 117, 

 plate 21, and which tissue I shall describe more fully in 

 treating on the subject of the dermal structures of the 

 Spongiadae. 



There are two distinct purposes in the physiological 

 application of the ternate spicula; the simplest is that of 

 strengthening and connecting the dermal membrane 

 with the mass of the animal beneath. The second and 

 more complex one, is that of forming an internal reticu- 

 lating framework for the support within its areas of the 

 valvular tissues forming the bases of the intermarginal 

 cavities. These offices of the ternate spicula are not demon- 

 strated in an equal degree of perfection in all sponges in 

 which they occur. Where the organs which they subserve 

 are best and most abundantly developed, these forms 

 of spicula are found in the greatest quantities, and in 

 the most regular and perfect mode of arrangement, but 

 where the intermarginal cavities or porous areas are in a 

 less regularly developed state, they are deficient in a cor- 

 responding degree; thus evincing the design and purpose 

 of their structure and presence. The most perfect and 

 beautiful illustration of their physiological purpose, in their 

 first mode of application, is afforded by the dermal mem- 

 brane of Dactylocalyx Prattii. Here we find their radii, 

 as described above, overlapping each other longitudinally, 

 and cemented together by keratode, forming a continuous 

 and regular network, upon the upper surface of which the 

 dermal membrane reposes, and to which it is firmly united. 

 The mode in which the radii are united, and the material 

 with which they are cemented together indicate a unity of 

 firmness and elasticity in the living state that is truly 

 admirable ; and this mode of structure we perceive is 

 especially necessary to the action of the dermal membrane, 

 as the whole of the skeleton beneath is perfectly rigid and 

 inelastic. Thus while their shafts are deeply plunged in, 

 and firmly secured to, the immoveable mass beneath, their 



