468 



THE VOYAGE OF H.M.S. CHALLENGER. 



INFLUENCE OF THE NATURE OF THE GROUND. 



It is necessary to consider in the first instance what kind of ground or bottom is 

 favourable to the occurrence of Hexactinellids. For this purpose the difierent kinds of 

 sea-bottom may be noted, along with the number of dredgings or trawlings on each 

 ground. Those searches must then be noted which yielded any Hexactinellida, so that 

 their proportion to the total number for each kind of bottom may be estimated. 



Then the proportionate abundance of species on each kind of bottom must l^e collated 

 with the number of dredgings or trawlings. In Table IX. (p. 470) the data for this 

 purpose are given and the general results may be summarised thus : — 



From this summary it is seen that the smallest number of localities yielding Hexac- 

 tinellida occurred on those bottoms which are described as green mud and red mud. A 

 somewhat small number were also dredged from Coral mud, volcanic mud, and red clay, 

 and then from Globigerina ooze and mud (including blue mud), on all of which the 

 percentage of Hexactinellid localities is over twenty-two. The same may be said of the 

 sandy and rocky ground. The localities on Pteropod ooze were somewhat more 

 abundant, viz., 30 per cent., and on Eadiolarian ooze yet more so, viz., 50 per cent. 

 The highest numbers refer to gravel and stones, 66 '6 per cent., and to Diatom ooze, 

 75 per cent. 



The statistics are somewhat different when the number of species found on the different 

 bottoms is taken into consideration. Here again green mud and red mud yielded the 

 smallest percentage of forms, viz., 5 '9 per cent, and 18*2 per cent, respectively. Then 



