40 DISCOVERY REPORTS 



indicate from which stage they moult. Whatever they are, the evidence they provide, 

 in conjunction with the "irregularly" moulting individuals, shows in the clearest way 

 that in this species development is not by a well-defined progression of stages in 

 which pleopods are successively added. 



It may be well here to quote Rustad (1930, p. 45) in his description of the larval stages 

 of Euphausia frigida and Thysanoessa macrura. He says: " Development seems to pro- 

 ceed rather schematically up to and including the Calyptopis stages. From the Furcilia 

 stage and upwards it seems on the other hand that there is greater room for action of in- 

 dividual variation. Certainly we observe a clearly pronounced main line which finally 

 leads up to the fully developed adults, but it is impossible to demonstrate any absolute 

 or fixed relation in degree of development between the different organs, and each ' stage ' 

 therefore covers a rather broad range of variation in degree of development and form of 

 the single appendage or organ." These remarks should be borne in mind, for they are 

 equally applicable to the development of Euphausia superba both with regard to the 

 Furcilia stages already discussed and those which are to be considered in the next 

 section. 



Macdonald (19276) quoting Gurney ( 1 924) states that ' ' Among Crustacea, continuous 

 larval development is a primitive feature, whereas a marked metamorphosis is charac- 

 teristic of more highly developed forms." For this reason Macdonald says "the above 

 observations are interesting as they suggest tentative steps in an evolutionary progress in 

 the order Euphausiacea towards reduction in the number of larval stages. They also 

 suggest that in those arthropods in which the life history consists of a few pronounced 

 stages these are to be regarded not as having evolved independently from a continuous 

 life history, but rather as the survivors of a once greater number of successive stages." 



In the light of the evidence provided by Euphausia superba and the interpretation of 

 known larval histories in relation to this I consider that it is misleading to talk of a ten- 

 tative reduction in the number of larval stages. This conception is based on the assump- 

 tion that each variety of pleopod development found indicates a larval stage. It implies 

 that as a primitive phase in the evolutionary history of this group the larvae went 

 through this succession of numerous well-marked changes in form, which is of course 

 contrary to the idea of continuous development. In primitive development one would 

 expect to find no well-marked coincidence of ecdysis with a fixed degree of development. 

 Metamorphosis, as already stated, is a characteristic of highly developed forms. The 

 Euphausiacea should be regarded as coming somewhere between the two extremes, they 

 are arthropods in which "continuous" development is giving way to metamorphosis — 

 already well defined in the Nauplius, Metanauplius and Calyptopis stages, less well de- 

 fined in early Furcilia by the presence of dominant forms, and still less intelligible in 

 later development. This developing metamorphosis is not essentially a reduction in the 

 number of successive stages, but is being brought about by the larvae tending to moult 

 into forms showing a certain degree of development in preference to a lesser or a 

 greater degree. 



Continuous development is not an altogether appropriate term to apply to the de- 



