GENERAL INTEREST OF SPONGES. 119 



J/isiory.—The Sponges, as one would expect, date back nearly to the 

 beginning of our geological record, for remains of a flinty form {Proio- 

 fiongia) have been discovered in the Cambrian rocks. Thence onwards 

 they are almost always represented. Remains of calcareous sponges are 

 almost confined to one peculiar set of large forms (Pharetrones), which 

 arerepresented in the Devonian and several succeeding epochs. Schulze 

 divides the sponge branch' of the zoological tree into three, the 

 calcareous forms to one side, the Hexactinellid — with triaxial flinty 

 spicules — to the other, and between these two — the flinty sponges 

 , whose spicules have four axes (Tetraxonia) or one (Monaxonia), and, 

 finally, the fibrous forms without spicules. Almost all zoologists regard 

 the Sponges as the simplest descendants of the primitive many-celled 

 animals, and there is also agreement that the sponge branch is a side 

 offshoot leading up to nothing else. 



Relation to other Organisms. — Sponges are living thickets 

 in which many small animals play hide-and-seeL Some of 

 the associations are practically constant and harmless, but 

 some burrowing worms do the sponges much damage. 

 Spicules, and offensive taste or odour doubtless save 

 sponges from being more molested than they are. On the 

 other hand, some sponges are borers, and others smother 

 forms of life as passive as themselves. Several crabs are 

 masked by growths of sponge on their shells, and the free 

 transport is doubtless appreciated by the sponge — till the 

 crab casts its shell. Within several sponges, minute Algae 

 constantly live, like the " yellow cells " within Radiolarians, 

 in mutual partnership or symbiosis. 



General Interest. — Sponges have been puzzles for centuries. 

 Neither zoologists nor botanists would have them. Peyssonel 

 regarded them as worm-nests, for were there not worms 

 inside? Lamarck thought they were colonies of polypes, 

 though the polypes were not to be seen ; the popular mind 

 classed them with seaweeds. A great step was made when 

 Robert Grant first detected how the water-currents went in 

 and out ; since then our understanding of sponges has been 

 rapidly progressive. They claim our interest, because they 

 are the lowest many-celled animals, because they show tissues 

 in the making, and because of the frequent beauty of their 

 hard parts. The practical utility of a few soft forms is 

 familiar. 



