CH. VIIl] A CENSUS OF THE SEA 167 



about in the sea, or live permanently attached to the bottom. A 

 catch made with any kind of plankton net will usually contain a 

 number of such eggs and larvae. For a time these drift about as 

 constituents of the planktonic life of the sea, but at some stage in 

 their life-history they abandon this transitory habit to take up 

 their definitive mode of life among the benthos or nekton. 



These transitory planktonic organisms are never so abundant 

 as the three groups mentioned above. If a tow-net is used in in- 

 shore waters sometimes a haul will be made which consists almost 

 entirely of the larvae of some invertebrate benthic animal, but 

 such collections are not often made. One may now and then find 

 the tow- net to contain only the zoea or megalopa larvae of crabs, 

 or the plutei of some sea-urchin, or the veliger larvae of some 

 mollusc. Thus Hensen records some hauls in the West Baltic 

 which indicated that the sea in the neighbourhood of the place 

 where the net was used contained some 170,000 larvae of the 

 common mussel {Mytilus edulis) per square metre of surface. 

 Sometimes these catches of larval planktonic animals may give us 

 some indications of the density, on the sea bottom, of the adult 

 creatures which gave birth to them. If, for instance, it had been 

 known what Avere the exact limits of distribution of the shoal of 

 Mytilus larvae to which I refer above, it would have been possible 

 to calculate their approximate number ; and if it had been known 

 what was the average number of eggs spawned by an adult mussel, 

 and if the probable destruction, by natural enemies, of these 

 larvae could have been estimated, then it might have been possible 

 to estimate the number of adult mussels on the sea bottom of the 

 region inhabited by the larvae. The possibility of such estimations 

 of the density of nektic and benthic animals is the chief object 

 of the quantitative study of the larvae of the plankton. 



Density of fish-eggs and larvae. Hensen's investigations of 

 the density of fish ova are among those results of quantitative 

 plankton work which have been most discussed in this country. 

 Because of their economic importance the fishes have been studied 

 more carefully than any other marine animals, and we know a 

 great deal about the life-history of the more common species. We 

 know, for instance, the average numbers of eggs annually spawned 



