118 



THE VOYAGE OF H.M.S. CHALLENGER 



occurrence of the families, genera, and species according to latitude in the northern 

 and southern hemispheres is shown : — 



This table shows that the numbers of pelagic Tunicata in the two hemispheres 

 are nearly equal. In this respect, as in their wide and fiiirly uniform distribution, 

 they form a contrast to the fixed Ascidians.' As is seen from the line of totals in 

 the table, the occurrences of the species curiously decrease in number in going north 

 from the equator, and increase in going south from it. But I do not lay stress 

 upon those numbers, as various cii'cumstances may have affected the numbers of 

 pelagic Tunicata collected in the diflTerent regions. 



1 See for Simple Ascidians this Report, Part I. p. 263 ; for Compound Ascidians this Report, Part II. p. 3G7. 



