PLANKTON 237 



Podon and Evadne are small Crustacea allied to Cope- 

 poda, wMcli may occur as dense local swarms in sum- 

 mer, and are an important element of the food of young 

 fishes. 



Oikopleura is a minute, pelagic, highly- organized animal, 

 related to the sedentary Ascidians of the benthos, but 

 having a locomotory tail provided with a rudimentary 

 backbone (notochord) and remaining free-swimming 

 throughout life. It is abundant in our seas at all times of 

 year, and is commonly known as Appendicularia. 



In addition to these and many other adult organisms, 

 there are in the plankton immense numbers of the eggs, 

 embryos, larvse and free-swimming stages of most of the 

 fixed and crawling animals, such as zoophytes, starfishes, 

 worms, crabs and molluscs, on the bottom. It is evident 

 then, even from this brief survey, that the plankton may con- 

 tain representatives of almost all kinds of marine organisms 

 and may be immensely varied both in amount and nature 

 at different localities and times of year. 



We now return to the methods of capture, and the investi- 

 gation of the problems plankton presents to the oceanog- 

 rapher, in its distribution both horizontally and vertically 

 and in its seasonal and other variations. 



Let us consider one or two pubhshed examples of the 

 problems in the economics of the sea which Hensen and his 

 feUow-workers undertake to solve by their quantitative 

 methods : — 



From certain samples obtained in the west Baltic it was 

 calculated that every square mile contained 80 to 100 biUion 

 Copepoda, and from the relative proportions of eggs, larvae 

 and adults it was deduced that for the sixteen square miles 

 of a certain fishery district the annual consumption of Cope- 

 poda must be 15,600 billions, and that consequently that 

 locahty supports Copepod-food sufficient for 534 milhon 

 herrings of an average weight of 60 grammes. 



Then, again, we are told that Brandt found about 200 



