242 GEOGRAPHICAL AND GEOLOGICAL DISTRIBUTION. 



hundred and sixty fathoms) the walls have almost entirely disap- 

 peared, so that the terminal opening forms a fissure in the oral disk, 

 the last remains of the tentacle being represented by a circular 

 margin surrounding the fissure, and so we come finally to the genus 

 Liponema (eighteen hundred and seventy-five fathoms), in which 

 the points at which the tentacles were actually placed are merely 

 indicated by openings in the oral disk." The very limited number 

 of specimens obtained by the expedition prevents any definite con- 

 clusions being arrived at as to the horizontal distribution of the in- 

 dividual species. The majority of the forms, as Anemone, Actinia, 

 Metridium, &c., inhabit a superficial zone, especially the rock-pools, 

 situated at about low-water mark, where they attach themselves to 

 some foreign body by means of their muscular pedal-disks ; others, 

 like Peachia, Edwardsia, Cerianthus, and Halcampa, lie more or 

 less buried in the sand, while a very limited number are free-swim- 

 ming or pelagic (Arachnactis). No fossil remains indisputably 

 belonging to this group have as yet been discovered. 



Much the larger number of coral-depositing zoophytes of mod- 

 ern seas belong to the group of the Madreporaria, or, as they are 

 frequently termed, from their dermal corallum, the Zoantharia 

 sclerodermata, as distinguished from those which, like the "black 

 corals " (Antipathidae), secrete an internal corallum or basal skele- 

 ton (Zoantharia sclerobasica). Two divisions of these corals may 

 be conveniently recognised, the " solitary " (whether simple or 

 compound) and the "massive," or reef -building corals, the former 

 of which are essentially deep sea forms, rarely coming within the 

 littoral zone, while the latter are just as distinctively shallow-water 

 forms, extending from low-water line to twenty or twenty-five 

 fathoms. 



The deep-sea corals are spread throughout nearly the whole of 

 the oceanic expanse, from the confines of the frozen sea on the 

 north to the Antarctic barrier on the south, and from very nearly 

 the surface of the water to depths of at least twenty-nine hundred 

 fathoms. The distribution of the individual species appears to be 

 largely, if not almost altogether, independent of considerations 

 connected with temperature. Sars obtained Fungiacyathus fragilis, 

 a member of the family Turbinolida?, off the Loffoden islands, from 

 a depth of three hundred fathoms. Bathyactis symmetrica, appar- 

 ently the most widely distributed of all known corals, has been 



