452 



DISTRIBUTION IN SPACE AND TIME 



The larvae of numerous Molluscs simply swarm in the surface 

 waters of the sea. 



Plankton Insects {Insccta). — Although Insects are essentially 

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



open sea. 



Plankton Crusta- 

 ceans iyCrnstaced). — 

 Many members of 

 this group, and in- 

 numerable crustacean 

 larvje, are amona 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 

 {Copcpoda, 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. 2S3). 



Plankton Annelids 

 ( Annelida ). — Some 

 members of this group 

 are specially adapted 

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

 {Tonwpteris) has elsewhere been described (see vol. iii, p. 22). 

 Plankton Echinodciins {Echinodcrmatci). — 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). 



Ncnicrtine J forms [lYcniertea) of the Plankton. — The larvae 



Fig. 1307. — Furk- Footed Criistace.-ins [Copt'poda'' with well-developed limbs. 

 I, OncEea venusta; 2, Copili.i vitre.i; 3, Caloc^l.tnus pavo. 



