316 TOILERS IN THE SEA. 



Certain tube-forming annelids (or worms) attach 

 themselves to the corallum of living coral, and in 

 some instances, so close is their relationship, that 

 they have been called " messmates." For instance, 

 in one species ( Mycedium fragile) the smooth under- 

 side of the coral, which is destitute of coral polyps, 

 is usually sprinkled with the tubes of a worm, in 

 company with encrusting Polyzoa, and mollusks. 

 The tubes of these worms are formed of carbonate 

 of lime, like the coral on which it establishes itself. 



" The calcareous tube, in which the worm lives, is 

 firmly united through most of its length to the 

 under side of the coral. Its terminal opening usually 

 lies near the rim of the saucer-like disk of the young 

 coral. In the growth of the rim this extremity of 

 the worm-tube is imprisoned by the increasing edge 

 of the disk, in such a way that it is wholly surrounded 

 by the live coral in the progress of its growth. The 

 growth of the worm-tube keeps pace with the ad- 

 vance of the coral about it, and, as it rises above the 

 upper surface of the coral, the opening into the tube 

 is kept uncovered by this growth, so that the head of 

 the worm, with its crown of branchiae, can always 

 have free communication with the outside water, not- 

 withstanding the tube itself is often wholly enclosed 

 in the calcareous secretions of the polyps. When the 

 growing coral covers the terminal opening of the 

 worm-tube the worm dies, but this rarely occurs, as 



