CH. Ill] LIFE IN THE SEA 69 



terminology, are important, though not relatively abundant con- 

 stituents of this category of marine life. 



\ 



The Crustacea form one of the great groups of the plankton, 



just as the same animals form perhaps the predominant inshore 



benthic population. We may attribute to the Crustacea the 



same predominance in the life of the sea that the insects possess 



on the land. Haeckel^ has pointed out that the struggle for 



existence has called forth the greatest variety in form and the 



most wonderful adaptations in structure, habits and life-history 



among the insects; and that just so the no less severe struggle in 



the sea has produced an even more wonderful diversity in form and 



habit among the Crustacea. It is among this group of animals 



that we find some of the most intelligent and powerful in the sea; the 



great lobsters and crabs for instance, which exhibit a high degree 



of specialisation in their structure and in their habits. Here too we 



encounter in the parasitic Crustacea some of the most degenerate 



and degraded of animals. The crab-barnacle (Saccidina), for 



instance, is by reason of its weird life-history one of the most 



wonderful animals in existence. Crustacea are always abundant 



and are quite universally distributed in the sea. Though they do 



not possess the astonishing fertility of the fishes nevertheless they 



exhibit a greater amount of specialisation in the varieties of 



devices for the care of offspring which makes for the greater 



fecundity of the class. 



To the benthos belong the lowest group of the Crustacea, the 



Cirripedes or barnacles, and the highest, the crabs, prawns and 



their allies. All these produce pelagic larvae which take a very 



important place in the plankton. The barnacles {Balanus) which 



form so abundant a part of the littoral fauna produce incredibly 



great swarms of larvae (Nauplii) which sometimes appear in the 



plankton to the exclusion of everything else, and for a time these 



nauplii drift about and then undergo metamorphosis into the 



Cypris stage and settle down on almost any object in the sea as 



the familiar little acorn-shells. So also with the crabs and their 



relations, the prawns and lobsters : these in their adult stages are 



comparatively large animals and produce great numbers of eggs. 



1 In the Plankton-Studien. 



