92 BRITISH SPONGES: 



the surface, because the necessity of ejectmg the excremential 

 matters to a distance does not exist. This is to bestow a fore- 

 sight and instinct on the sponge, which even the followers of 

 Lamarck would hesitate to give it, and which we may safely de- 

 ny it to be possessed of. The form of the oscula depends en- 

 tirely on the texture of the species and on the force of the efflu- 

 ent currents. If the texture is loose and fibrous it yields easily, 

 and the oscula are level or nearly so ; if more compact the skin 

 is pushed beyond the surface into a papillary eminence ; and if 

 too firm and dense to yield to the pressure behind, the oscula 

 fall again into a level condition. They are also liable to be mo- 

 dified in some degree by external forces, for the littoral sponge, 

 which, in a sheltered hollow or fringed pool, will throw up cra- 

 ters and cones from its surface, may be only perforated with 

 level oscula when it is swept over and rubbed dowTi by the waves 

 of every tide. 



That some Halichondriae are propagated by ciliated coloured 

 ova is proved by Dr Grant, but I cannot concur in the conclu- 

 sion that all the species have the same genesis. Siirely were it 

 so, ova would sometimes have been found in some of the nume- 

 rous specimens of the many species which I have examined, but 

 evidence of their presence has been in vain sought for. The 

 growth of the species is certainly rapid, and many of them are 

 annual. 



* Texture fibro-reticular ; the spicula imbedded in the fibres, 



1. H. PALMATA, arhuscular, irregularly branched, the 

 branches broad and compressed, ivith scattered oscula ; tex- 

 ture rather coarse, rigid, and tough ; spicula short, dotible- 



pointed. 



Plate II. 



Spongia palmata, Sibb. Scot. ill. ii. lib. iv. 55- Ellis and Soland. 

 Zooph. 189, pi. 58, fig. 6. Turt. Gmcl. iv. 659. Turt. Brit. 



