PHYLUM CCELENTERATA 



209 



Crustacea, which have their abode in the enteric cavity. In this 

 case the Fish secures shelter in a place where it is very unlikely to 

 be disturbed, and the two animals are strictly commensals or " mess- 

 mates " since they share a common table. A somewhat similar 

 instance is furnished by the Blue Coral (Heliopora), already referred 

 to more than once. The corallum contains, not only the apertures 

 for the polypes and siphonozooids, but also tubular cavities of 



. 



FIG. 157. Adaxnsia palliata, four individuals attached to a Gasteropod shell inhabited by 



a Hermit-crab, nc. ac 1 . acontia ; gh. shell of Gasteropod. (After Andres.) 



an intermediate size, in each of which is found a small chaetopod 

 Worm, belonging to the genus Leucodore. As the polypes are 

 frequently found retracted at a time when the Worms are protruded 

 from their holes in search of food, it is not surprising that the 

 latter should have been credited with the fabrication of the coral. 

 Trapezia, a genus of Crabs, always lives in interstices of a par- 

 ticular species of Madrepore. 



The distribution of the Actiniaria is world-wide, and in 

 many cases the same genera are found in widely separated parts 

 VOL. I P 



