308 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



waters. Most of thern have fixed or resting bottom stages 

 in their life-history, and so fall into Haeckel's mero- 

 plankton, but some Neritic forms are holoplanktonic, 

 being permanently free. 



Table I gives a list of the species found during '09, 

 with the number of days in each month on which they 

 occur. It also shows the number of species present each 

 month, and the proportion of Oceanic species. It will be 

 seen that only in two months (January and December) do 

 the Oceanic forms outnumber the Neritic, while in July 

 the two are equal. For the other months the percentage 

 of Oceanic species ranges from 40 to 48'5. This would 

 tend to show that Port Erin Bay is rather more Neritic 

 than Oceanic in character ; that is, that in most months 

 more Neritic than Oceanic species occur. But that, 

 although true, is not the whole truth, since it recognises 

 in each month only the number of species that have 

 occurred, and not the total number of their occurrences, 

 nor the average number of times that Oceanic or Neritic 

 species occurred in each month. 



Consequently, it is interesting to consider Table II 

 (p. 312), which gives the mean occurrence or average 

 number of days per month on which Oceanic and Neritic 

 forms, respectively, occurred, obtained by dividing the 

 sum of the number of days on which the various forms 

 (Oceanic or Neritic) occurred ( = total occurrences) by the 

 number of species. The factor thus obtained shows the 

 relative constancy of the two groups of species, and the 

 table shows that the average for the Oceanic forms is 

 much higher than for the Neritic, that is, that the 

 Oceanic forms are a much more constant feature of the 

 plankton. Further, we show the percentage that each 

 average number of occurrence-days is of the total possible 

 number of plankton-days in that month, and we are 



