Sponges. 49 



stiffen the skeleton) have no keratode ; others (those required for 

 the retention of prey) possess a large portion of this substance. 

 Spicules originally consist of two layers of membranaceous matter, 

 forming a hollow cone. Layers are added to this both externally 

 and internally as the spicule grows; in adult specimens, the 

 internal cavity is reduced to a line. 



The functions of the spicules differ widely. Often they form 

 the whole skeleton ; at one time like most beautiful lace, as in the 

 Eui)lectella, or \'enus' flower-basket ; at another time triradiate, 

 to interlace and leave free canals for the circulation of the water; 

 again, in Tethiura and Geodia, they are large, stout, and straight. 

 When they are accessory to keratode in forming the skeleton, they 

 are spiny, to give them a firm hold. These skeleton spicules are 

 found even in the young gemmules before they become detached 

 from the parent. 



Another kind, defensive spicula, are divided into two classes: 

 those of the exterior, and those of the interior. The former 

 present a double line of defence ; large projecting spicules inclined 

 at every angle against important enemies, backed up by spicula 

 all but buried in the sarcode, which effectually stop the more 

 insidious attacks of the smaller annelidse, etc. Another species 

 attracts fishes by the tempting look of the sarcode, to make a 

 dinner off it ; the depredator however closes his mouth on in- 

 numerable imbedded points, instead of a savoury morsel. 



Internal Defensive Spicula are straight, and line the canals in 

 a sloping direction, so as to impale an intruder, but let a retiring 

 foe escape unharmed. It is not always however that they are 

 allowed thus to escape. 



We often find in the internal canals two kinds of spicula mixed: 

 one furnished with hooks, to retain the prey ; the other sharp and 

 pointed, so that the struggles of the victim would redouble its 

 wounds ; while severe laceration is provided for by the spines 

 which stud its surface. Another kind (from the Atlantic) shows 

 two sharply-bent hooks united at the ends of a short shaft ; the 

 inside edges of the hook and the opposite part of the shaft are 

 sharp, while the bend of each hook and the middle of the shaft are 

 round. These spicules are fastened to the skeleton by the bend 

 of one of the hooks, each of which, like a pair of hand-shears, 

 would obviously retain and mangle its prey. The laceration is 

 caused in order to make the victim give up its juices as rapidly as 

 possible. 



Sarcode is a semitransparent gelatinous substance, variable in 

 colour, and insoluble in water. When exposed to air it rapidly 

 dries, but is restored again by immersion. It seems to be destitute 

 of feeling, since it does not contract or move when cut or burnt. 



E 



