MARINE DENIZENS OF THE REEF. 153 



The richly impurpled apertures of these tubes (all of equal height) 

 are separated from one another by the fraction of an inch; but they are 

 cemented at their partly coiled bases, and the interstices speedily become 

 clogged with sediment, forming a solid mass of lime. At high tide, when 

 myriads of tentacles can be seen through the wonderfully clear Gulf water, 

 protruding from the submerged and bristling surface, grappling industri- 

 ously, but with feathery grace, for their microscopic food, this serpula 

 colony becomes an animated and beautiful picture. 



I suspect that these worms survive only one year, and then dying leave 

 their indestructible cases to serve as the foundation upon which their 

 progeny may rear a new tier of tubes. Thus, by the additions of succes- 

 sive generations (as in the case of the coral growth, but through a differ- 

 ent history) this worm-structure increases into an extensive mass of heavy 

 rock. I have seen pieces many yards square and two feet or more thick. 



Growing irregularly, crannies afford a haunt for many species of mol- 

 lusks and crustaceans that like to hide away in holes ; and the mass is 

 further enlarged by the growth upon it of bunches of oysters, and by the 

 filling of the interstices with sand and broken shells which become solidi- 

 fied along with the worm-tubes through the production of a native cement. 

 Thus millions of tons of limestone, most useful for building purposes, are 

 each decade added to the Floridan coast by despised worms. 



Attracted by the excellence of the hiding-places offered, and by the 

 abundance of "small deer" lurking there, many predatory sorts of aquatic 

 animals come to the mangrove roots in search of food — conchs, whelks, 

 boring sea-snails, crabs of several species, and mollusk-eating fish like 

 the sheep's-head. Where there is teeming life, death must be frequent. 

 Thousands of empty shells and fleshless skeletons sink into the animated 

 ooze along the edge of the reef, and rapidly fill it up until the water 

 no longer covers it, except at the highest tide, and then leaves behind an 

 important toll of drift-wood, so that the adventurous, water-loving man- 

 groves must push their roots farther and farther into the sea in order to 

 find the best conditions for growth. 



Meanwhile a similar process has been raising the centre of the island. 

 Decay of grass and salt weeds, mangroves and drifted wood, finally lifts 

 a surface permanently above the water. Huge flocks of water-birds daily 

 alight upon it to rest and feed, and their droppings increase and enrich 

 the soil. Various seeds are blown or floated from the main-land and con- 

 tribute to its stock of vegetation ; various land animals — chiefly reptilian 

 — make the new key their home, die and are buried there. To the simple 

 mangrove swamp thus succeeds an intermixture of oak, pine, and pal- 



