72 THE DEPTHS OF THE SEA. [chap, ii, 



heads of five-rayed spicules. One ray of each 

 spicule dips directly into the body of the sponge, 

 and the other four, which are at right angles to it, 

 form a cross on the surface, giving it a beautiful 

 stellate appearance. The silicious rays of one star 

 curve towards and meet the rays of the neighbouring 

 stars, and run parallel with them. All the rays of 

 all the spicules are thickly invested with consistent 

 semi-transparent gelatinous matter, which binds their 

 concurrent branches together by an elastic union, 

 and fills up the angles of the meshes with softly 

 curved viscous masses. This arrangement of the 

 spicules, free and yet adhering together by long 

 elastic connections, produces a strong, flexible, and 

 very extensible tissue. The cylindrical oscular cavity 

 within the sponge is lined with nearly the same kind 

 of network. 



When the sponge is living, the interstices of the 

 silicious network are filled up both outside and in with 

 a delicate fenestrated membrane formed of a glairy 

 substance like white of egg, which is constantly 

 moving, extending or contracting the fenestrae, and 

 gliding over the surface of the spicules. This 

 ' sarcode,' which is the living flesh of the sponge, 

 contains distributed tlirough it an infinite number 

 of very minute spicules, presenting the most sin- 

 gular and elegant forms very characteristic of each 

 species of sponge. A constant current of water 

 carried along by the action of cilia passes in by 

 apertures in the outer wall, courses through the 

 passages in the loose texture of the intermediate 

 sponge-substance carrying organic matter in solution 

 and particles of nourishment into all its interstices, 



