SPONGID.E. 37 



fully investigated than that of most other members 

 of the group. 



8. §triieture of Tethya. — From the obser- 

 vations of Prof. Huxley it would appear that this 

 Sponge consists essentially of three distinct struc- 

 tures, viz. : — 



1. A central, whitish, spherical substance {fig. 7, 



6, X) ; composed of a granular mass, asso- 

 ciated with numerous cylindrical spicula. 



2. A yellowish red intermediate portion {fig. 7, 



h, 13) ; composed of a granular uniting sub- 

 stance in which ova and stellate spicula (a, k) 

 are embedded. 



3. A deep red cortical substance (6, a); con- 



sisting of two zones, which merge insensibly 

 into one another. Of these, the inner is 

 composed of closely interwoven bimdles of a 

 filjrous tissue, and contains only a small num- 

 ber of stellate spicules, whilst the outer zone 

 is dense, granular, " containing great num- 

 bers of crystalline spheres beset with short 

 conical spikes." 



The rod-like spicula which occur in the central 

 substance are usually so arranged as to form an 

 irregular network, becoming aggregated into bun- 

 dles as they approach the intermediate substance. 

 The several spicula contained in these bundles are 

 at first nearly parallel to one another, bub they 

 gradually diverge as they radiate through the 

 latter, terminating at length, in the cortical layer, 

 beyond which a small number not unfrequently 

 project. Numerous long solitary rods, in addition 

 to the bundles, also radiate through the interme- 



T> 3 



