ANCIENT KEEFS. 153 



the known fact that the practicable channels of entrance into 

 encircling reefs or atolls are usually to leeward. 



The winds and waves are singularly aided in grinding 

 down the corals into mud and fragments by the Scari and 

 Holothurice which haunt the reefs ; the former browsing 

 upon the living polyps, with their hard and parrot-like jaws, 

 and passing a fine calcareous mud in their excrements ; the 

 latter, more probably, swallowing only the smaller fragments 

 and mud, and, having extracted from them such nourishment 

 as they may contain, casting out a similar product. It is 

 curious to reflect upon the similarity of action of these worm- 

 like Holothurice upon the sea-meadows of coral, to that 

 which the Earthworms, as Darwin has shown, exert upon our 

 land-meadows ! 



In the Palaeozoic period reefs like those which have just 

 been described appear to have abounded in our own latitudes ; 

 and there is the most striking superficial resemblance be- 

 tween the ancient beds of calcareous rock which record their 

 existence, and the masses of coral limestone, hard enough to 

 clink with a hammer, which are now being formed in the 

 Pacific, by the processes of accumulation of coral mud and 

 fragments, and their consolidation by percolating water. 

 Closer examination, however, shows an important difference 

 in the nature of the corals which compose the two reefs. The 

 modern limestones are made up of Perforata, Millepores, 

 and Aporosa. The ancient ones contain Millepores, but usu- 

 ally neither Perforata nor Aporosa — both these groups being 

 replaced by the Pugosa, none of whose members (with some 

 doubtful exceptions) have survived the Palaeozoic period. 

 On the other hand, Palceocyclus and Pleurodictyon are the 

 only genera belonging to the Aporosa or Perforata, which 

 have yet been discovered in strata of greater than mesozoic 



The Ctenophora. 1 — These are freely-swimming marine 

 animals, which never give rise by gemmation to compound 

 organisms, and are always of a soft and gelatinous consist- 

 ence, their chief bulk being made up by the greatly -devel- 

 oped mesoderm. Many are oval or rounded (Peroe, Plenro- 



1 Allman (" Monograph of the Tubularian Hydroids," 1871, page 3) consid- 

 ers that the Ctenophora are more properly arranged among the Hydrozoa. I 

 confess, however, that I see no reason to depart from the conclusion to which 

 I was led by the study of the structure of Pleurobrachia, many years ago, that 

 the Ctenop&ora are peculiarly modified Actinozoa. 



