102 ANATOMY AND PHYSIOLOGY 



of the membrane of the diaphragm was always outward as 

 regards the cavity, so that when viewed from within it 

 appeared as a slightly funnel-shaped depression, the bottom 

 of which was conical. The cavities are lined by a smooth 

 and tolerably strong membrane, abundantly supplied Avith 

 slender fibrous tissue, disposed in nearly parallel lines at 

 right angles to the long axis of the cavity. 



The adaptation of the skeleton to the support of these 

 elaborately constructed organs is very remarkable. The 

 sponge is furnished abundantly with large expando-ternate 

 spicula, the radii of which are furcated at their apices. 

 They occur in a series of bundles; the long attenuated 

 shafts of each fasciculus approximate at their bases, and 

 diverge thence until the ternate head of each is about 

 equally distant from its surrounding neighbours, and the 

 extremities of the rays touch or slightly cross each other, 

 thus forming a beautiful and regular network, the meshes 

 being six- or seven-sided, according to circumstances. The 

 upper surfaces of the radii are firmly attached to or par- 

 tially imbedded in the under surface of the crustular stratum, 

 and the areas thus formed are occupied each with the 

 proximal valvular terminations of one of the intermarginal 

 cavities. 



The progressive development of these inhalant areas, 

 formed by combinations of the radii of the ternate forms of 

 spicula in different species of sponges, is very interesting. 

 In PacJiymatisma they are so indefinite that they can 

 scarcely be said to exist. The ternate spicula are few in 

 number, and very irregular in their mode of disposition, and 

 a faint indication only of their future regular combination to 

 form the dermal reticulation is apparent. In the more 

 highly organized genus Geodia we find them in different 

 species in progressive stages of combination, until, in G. 

 M'Andrewii and Barretti, the apices of the radii of the ter- 

 nate spicula are interlaced with each other, and a continu- 

 ous irregular network is formed, each area of which is filled 

 with the proximal termination of an intermarginal cavity. 

 In Dactijlocahjx Prattii, Bowerbank, MS., the structure 

 advances another stage towards perfection. 



