6 DISCOVERY REPORTS 



SYSTEMATIC IMPORTANCE OF THE LARVAE 



Perhaps the most interesting feature of the development of Sergestes is the striking 

 difference which exists between the larvae of the different species, while the adults are 

 often separable with difficulty. It may well be that knowledge of the larvae, when 

 complete, may give a better indication of the relationships of the species than the adult 



structure. 



The Elaphocaris in stages 2 and 3 may be divided into three types which are quite 

 distinct and have been called dohnii, ortmanni and hispida respectively (Gurney, 1924c, 

 p. 79). In all cases the carapace has the same number of processes, but they differ as 

 follows : 



(i) Dohrni type. Supraorbital, lateral and posterior processes with numerous long 

 lateral spines. 



S. atlantiais, cornutiis, arctiais, pectinatiis, sargassi, vigilax, armatus, diapontius, 



edwardsi. 



(2) Ortmanni type. Lateral and posterior processes without lateral spines, but with 

 long spines springing from the carapace at their bases. 



S. cormculitm. 



(3) Hispida type. Lateral and posterior processes without long spines. Sometimes 

 with long spinules at base. 



S. tenuiremis, robustiis, crassiis, prehensilis. 



The ortmanni type seems to be a derivative of the doJirni type rather than of the 

 hispida type, since the supraorbital processes have long spines as in the former. 



While S. corniculum and the species of the robustiis group have perfectly characteristic 

 forms of Elaphocaris, the dohrni type is found in a number of species which are not 

 supposed to be particularly closely related, and it is not possible to separate them into 

 groups on the structure of this stage. The Elaphocaris of S. arcticus, for example, is very 

 distinct in its form of eye and the peculiarly branched spines, but it is otherwise of 

 typical dohrni type. 



The Acanthosoma cannot be separated into types corresponding to the Elaphocaris. 

 One group comprising S. vigihix, diapontius, armatus and edwardsi is, however, charac- 

 terized by an Acanthosoma of distinctive form with very short telson (see p. 46). 



The species of the robustus group of which the development is known have Acantho- 

 somas with particularly long telson arms, but no other very distinct common characters, 

 thouo-h there appears to be a group of larvae resembling that of S. robustus which cannot 

 at present be identified (see p. 33). The Acanthosoma of S. arcticus more closely 

 resembles that of the robustus group than any other, though the points of resemblance 

 are not particularly significant. 



We have given evidence (p. 38) that there are at least three species of Acanthosoma 

 without posterior dorsal spine, and we consider that these larvae belong to species 



