45 2 



DISTRIBUTION IN SPACE AND TIME 



The larvae of numerous Molluscs simply swarm in the surface 



waters of the sea. 



Plankton Insects (Insecta). Although Insects are essentially 



land-forms, a few Bugs (e.g. Halobates] live on the surface of the 



open sea. 



Plankton Crusta- 

 ceans (Crustacea). - 

 Many members of 

 this group, and in- 

 numerable crustacean 

 larvae, are among the 

 most important plank- 

 ton animals. Some, 

 as the Swimming- 

 Crabs, may be of fair 

 size, but by far the 

 most dominant order 

 is that of the Fork- 

 Footed Crustaceans 

 (Copepoda, fig. 1307), 

 which are of great 

 economic importance, 

 because they constitute 

 the staple diet of Her- 

 rings and some other 

 valuable food - fishes 

 (see p. 283). 



Plankton Annelids 

 ( Annelida ). - - Some 



Fig. 1307. Fork-Footed Crustaceans (Copepoda^ with well-developed limbs. mPlTlbprS of fhi^ OTOlin 

 i, Oncaea venusta; 2, Copilia vitrea; 3, Calocalanus pavo. 



are specially adapted 



to a life in the surface waters, and one remarkable example 

 (Tomopteris] has elsewhere been described (see vol. iii, p. 22). 



Plankton Echinoderms {Echinodermata\ The curious larvae 

 of all sorts of Echinoderms are abundantly found in plankton at 

 certain times of the year, but the adult members of the group 

 seem little suited for this kind of life. A kind of Sea-Cucumber 

 (Pelagothuria), however, has acquired the necessary adaptations 

 for the purpose (see vol. iii, p. 24). 



Nemertine Worms (Nemerted) of the Plankton. The larvae 



