300 POLYCHAETA 



As for hathy metrical distribution, 1 many genera occur at all 

 depths, though Polychaetes appear to be most abundant, as far as 

 we know at present, in " shallow water " that is, down to twenty 

 fathoms or so ; but this may be due to the greater facility of 

 collection on shore and in these slight depths, for the " Challenger " 

 obtained considerable numbers of new species at greater depths. 



The " deep-sea " forms are chiefly tubicolous, and since these 

 tubes are fixed and partially embedded in the bottom, probably 

 comparatively few are brought up. Some genera occur at very 

 great depths ; thus the Terebellid Zeaena abyssorum and the 

 Serpulid Placostegus benthalianus were brought up from 3125 

 fathoms the greatest depth from which Polychaetes were 

 obtained by H.M.S. " Challenger " ; and it is interesting to note 

 that species of each of these two genera occur in shallow water, 

 the Serpulid being represented in our own coast fauna by P. 

 tricuspiclata. 



Amongst our own fauna, a few examples may be given of the 

 " replacement of species." 2 The littoral Sthenelais boa is repre- 

 sented by S. limicola in deeper water ; Sabellaria cdveolata by 

 S. spinulosa; Polynoe imbricata by several deep-water species. 

 Similarly with genera : the littoral Po?natoceros is replaced by 

 Serpula in deeper water ; and the Hesionid Psamathe by Castalia. 



The limitation of species to certain regions, or to certain 

 depths of an ocean, may appear at first sight peculiar, in view 

 of the unrestricted communication between all its parts; but 

 fhere are as efficient " barriers " there as on land, for generally 

 a particular worm can live only in a certain temperature and at 

 a certain pressure, and is dependent for its food on particular 

 organisms, which in their turn depend on the depth and its 

 accompaniments. It is, in fact, so much the more peculiar that 

 certain species are more or less cosmopolitan, or occur at widely 

 distant points. It is less peculiar, of course, to find different 

 species of the same genus at different . depths or in different areas, 

 for any slight variation in a species advantageous to new con- 

 ditions would readily be fixed, and give rise to a new species. 



The distribution of the Polychaeta depends probably on the 

 pelagic larvae, which are carried by currents from one part of an 

 ocean to another. There can be little doubt that many Poly- 



1 "Challenger" Reports, loc. cit. p. xxx. 

 2 See Hornell, Fauna of Liverpool Bay, Report III. 1892, p. 126. 



