PRE-DETERMINATION IN NATURE 337 



The sponge, in order to support its delicate protoplasmic 

 structures, must have a skeleton. In modern times we find 

 these creatures depositing corneous or horny fibres, as in the 

 common washing sponges, or forming complex and beautiful 

 structures of needles, or threads of silica or calcite, and they 

 seem from the first to have been able to avail themselves of 

 all these different materials. The oldest species that we know 

 had silicious or calcareous skeletons, though some of them 

 must also have had a certain amount, at least, of the ordinary 

 spongy or corneous fibres. But the most astonishing feature 

 in what remains of their skeletons, flattened out as they are on 

 the surfaces of dark slaty rock, is the manner in which they 

 worked up so refractory a material as silica into fibres like spun 

 glass rods and crosses, and built these up into beautiful basket- 

 like forms, globular, cylindrical or conical. It was necessary 

 that they should fix themselves on the soft muddy bottoms 

 on which they grew, and to this end they produced slender 

 silicious fibres or anchoring rods, which, fine though they were, 

 had the form of hollow tubes. Sometimes a single rod sufficed, 

 but in this case it had a crosslike anchor affixed to its lower 

 end, to give it stability. Sometimes there were several simple 

 rods, and then they were skilfully braced by spreading them 

 apart at the ends, and by flattening their extremities into 

 blades. Sometimes four rods joined in a loop at the end gave 

 the required support. Some larger species wound together many 

 threads like a wire rope, and even added to this flanges like the 

 thread of a screw, anticipating the principle of the modern 

 screw pile. 



The body of the sponge must be hollow within, and must 

 have a large aperture or opening for the discharge of water, and 

 smaller pores for its admission. Various general forms were 

 adopted for this. Some were globular, or oval, or pear-shaped ; 

 others cylindrical, concave, or mitre-shaped. To give form and 

 strength to these shapes there were sometimes vertical and 



