216 



MARINE ANIMALS 



scatter in all directions. As if there were not enough natural hollows, 

 a whole series of forms bore into the coral. The examination of a soli- 

 tary madrepore, located at a distance from the reef on sand bottom, 

 shows how attractive it is to other animals. The whole animal popula- 

 tion of the surrounding area will be concentrated on and in it, and 

 life abounds there in the midst of an otherwise impoverished area. 77 



There is scarcely a benthic group of animals not represented in the 

 coral communities. It must suffice to emphasize the most important 



common characteristics. As in general 

 on solid bottom, numerous sessile animals 

 are present. In addition to the reef-build- 

 ing corals, corals which merely inhabit 

 the reef without contributing to its struc- 

 ture are present, such as the solitary 

 proteoid Fungia. Sea anemones are nu- 

 merous, among them the gigantic dis- 

 cosomatids with an oral surface 0.5, 

 even 1.2 m. {Stoichactis) , in diameter. 

 Gorgonids may be abundant, especially 

 in the West Indian reefs; they form a 

 special gorgonid zone, below 10 m., in 

 the Tortugas. Sponges of all sizes occur, 

 up to the great Neptune's cup, Poterion. 

 The lamellibranchs are here character- 

 ized by special thickness of shell, among 

 them oysters, pearl oysters, and the 

 spiny-shelled Spondylus; the gigantic 

 Tridacna, which reaches a weight of 

 250 kg., is an index species of the Indo- 

 Pacific reefs. A number of coral-boring 

 forms are represented, such as Lithodo- 

 mus, and others, like Coralliophaga, are grown over by coral. Nu- 

 merous gasteropods are sessile, such as Leptoconchus, living in holes 

 open only by a crack, obviously capable of increasing the holes as 

 they grow. Magilus is remarkably transformed by its life in the coral ; 

 beginning as a snail with ordinary spiral shell, in cracks in the coral, 

 it changes its direction of growth and extends the shell more or less 

 in a straight line, as it becomes overgrown, in order to maintain its 

 contact with the outer world, filling the older portion of the shell 

 with lime (Fig. 15, p. 168). Serpulid worms are abundant on reefs and 

 may play a part in their structure. 



The snails of the coral biotope are remarkable. The distribution of 



Fig. 45. — Palolo worm (Eu- 

 nice viridis). The thick an- 

 terior end remains in the reef, 

 the more slender posterior por- 

 tion breaks off and swarms in 

 the open water as the palolo. 

 After Woodworth. 



