28 MR. R. GURNEY ON THE [May 12, 



are six-jointed, one might assume that a seventh is fused with the 

 body- wall, in Avhich case the gills might be interpreted as arthro- 

 branchs or podobranchs at will. Still, in the common acceptance 

 of the term, they certainly seem to me to be properly designated 

 as pleurobranchs. 



I have been unable to detect at this stage the small arthro- 

 branch found in the last larva and in the adult on the third 

 maxillipedes. 



The first postlarval stage difiers from the adult in that there is 

 but a single median tooth on the dorsal surface of the carapace, 

 instead of three teeth transversely arranged. For this reason 

 the young shrimp at this period is hardly distinguishable from 

 C. nanus ; but the latter is somewhat smaller, and the single 

 median tooth of the carapace seems to rise from a short median 

 ridge. 



Sars figures a small posterior tooth in this ridge, corresponding 

 to the second tooth of the adult, but I have been unable to detect 

 it in my specimens. The possession of peculiarly distinct larval 

 forms with sudden transition to a uniform postlarval type is one 

 of the most remarkable features in crangonid metamorphosis, and 

 the two forms described are no exception to the rule. 



Conclusions. 



Our knowledge of Decapod metamorphosis is for the most part 

 very fragmentary, and for this reason it has been impossible to 

 use the larval stages as evidence of relationship. We have, how- 

 ever, an exceptionally full knowledge of the metamorphosis of the 

 Crangonidee ; and it will be interesting to see if a comparison 

 of the larv0e will throw light on the classification of the group. 

 The eleven species of Crangonidfe which have been recorded as 

 British have been divided up among five genera : — Crangon Fabr., 

 Gheraphilus Kinahan, jEgeon Risso, PontopMhis Leach, and 

 Sabinea Owen; though Ortmann in his revision of the family 

 joins Gheraphihis, jEgeon, and Pontojjhilus into one genus Ponto- 

 philus. The genus Sabinea may here be left out of account. 



Looking, then, at the larvje of the species that have been 

 described, we find them falling, as it seems to me, naturally into 

 three groups : — 



1. C. vulgaris and C. allmamii : characterised by a one- jointed 

 maxilla-palp and the absence of an exopodite on the second leg 

 in the Mysis-stage. 



2. C. trispinosus, G. namis, C. echinulatus, and C. fasciatus : 

 characterised by their two-jointed maxilla-palp, possession of five 

 pairs of exopodites in the Mysis-stage, form of the rostrum, and 

 arrangement of the abdominal spines. 



3. C. spinosus and C. norvegicus : distinguished from the second 

 group by their extremely elongated body form, shape of the 

 rostrum, possession of a single long median spine on the third 

 abdominal segment, and by the form of the tail-plate. 



