LARVAE OF DECAPOD CRUSTACEA 



PART VI. THE GENUS SERGESTES 



By R. Gurney and M. V. Lebour 

 (Text-figs. 1-56) 



INTRODUCTION 



THE main facts in the development of the Sergestidae are now fully known, and the 

 history of the progress of knowledge has been summarized by Wasserloos (1908) 

 and by Hansen (1922). Of two species only, S. arcticiis and S. prehensilis (by Wasserloos, 

 1908 and Nakazawa, 1915, 1916), are all the stages known with certainty. While the 

 Acanthosoma stages do in some cases offer characters which make it possible to link 

 them up with the Mastigopus and so with the adult, the Elaphocaris stages are so com- 

 pletely unlike the Acanthosoma that it is generally impossible to connect them with later 

 stages when only preserved material is available. 



Among the larvae in the Elaphocaris stage certain types are distinguishable (Gurney, 

 1924c), and it is of real importance that they should be identified, if possible, since they 

 may help to point to a satisfactory subgeneric grouping of the adults. While it is occa- 

 sionally possible to link together a series of larvae in a plankton sample with some 

 approach to certainty, a real advance can only be made where living material is available 

 in quantity, and the larvae can be kept alive for moults from one stage to another. The 

 Biological Station in Bermuda offers ideal conditions for such work. In the spring of 

 1935 one of us (R. G.) devoted much time to study of Sergestid larvae and obtained 

 a number of moults, but the results were not sufficiently complete for publication. In 

 1938 both of us spent part of the summer in Bermuda, and one of us (M. V. L.) con- 

 tinued to work there for a whole year. For a successful issue to such research a regular 

 supply of living plankton from the open ocean down to depths below 200 metres is 

 necessary, and samples from deep water have been regrettably few. Still, the material 

 that has been obtained permits of connecting up the early stages with the Mastigopus 

 in a number of species, partly by means of colour resemblances and partly by moulting. 

 On the whole the larvae live well in the laboratory ; but there are difficulties to overcome. 

 It is generally impossible to make any adequate examination of a larva which it is 

 intended to keep alive, and the moulted skin is so fragile that it is of very little use for 

 subsequent identification. On two occasions in 1935 a larva actually moulted while a 

 drawing was being made of it. 



A large amount of material from the ' Discovery ', mainly of Mastigopus stages, has 

 been studied, and some of this is dealt with in this report. This material includes some 

 larvae which have not been seen at Bermuda. 



