REPORT ON THE TUNICATA. 273 



V. — Between 2000 fathoms and 3000 fathoms seven species were found, viz.: — 



CiiJcolusmurriiiji. 

 moseleyi. 

 Styela bythia. 



squamosa. 

 Corynascidia suhmi. 

 Abyssascidia wyvillii. 

 Hypobythius calycodes. 



In this list five genera and two families are represented. 



From these lists it appears that Simple Ascidians are much more common in shallow 

 than in deep water, and that comparatively few — twenty species in all — extend into 

 the abyssal zone, while more than twice as many species are found between the shore 

 and 50 fathoms as between 50 fathoms and 500. 



These lists, however, do not represent accurately the whole state of affairs, as they do 

 not take into account the greater facilities for collecting in shallow water, nor yet the 

 relative numbers of the deep and the shallow-water dredgings performed during the 

 voyage. 



Hence the following list, showing the number of dredgings taken by the expedition 

 in the different zones, and the proportion of them at which Simple Ascidians were found, 

 is necessary, in order to give a complete idea of the bathymetrical distribution : — 



In 32 dredgings at from 0-50 fathoms, Ascidias Simplices occurred 10 times, 1 or at 31 per cent, of the Stations. 

 „ 51 „ 50-500 „ „ 10 „ 20 



„ 23 „ 500-1000 „ „ 2 „ 9 



„ 94 „ 1000-2000 „ „ 3 „ 3 



„ 1G1 „ 2000-3000 „ „ 5 „ 3 



The column of percentages brings out clearly that although Simple Ascidians extend 

 into very deep water, and are fairly well represented in the abyssal zone, still they are 

 chiefly a shallow- water group, and attain their greatest numerical development immedi- 

 ately around coasts in a few fathoms of water. 



It seems impossible to establish any relation between the occurrence of Simple 

 Ascidians and the nature of the bottom. The shallow-water forms appear to affect mud 



1 In addition to this, Simple Ascidians were collected from depths le a than 50 fathoms al nine 1 calities which 



are not given in the list of Stations, viz. :— Bermuda, Bahia, Simon's Bay, Port Jackson, Kandavu, Bauda, Hoi - 

 Koiifr, Yokohama, and Tahiti. 



