12 THE DEPTHS OF THE SEA. [cHaP. 1. 
consists of a complicated network of the cross-like 
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 con- 
stantly moving, extending or contracting the fenestree, 
and gliding over the surface of the spicules. This 
‘sarcode,’ which is the living flesh of the sponge, 
contains distributed through 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 
