OCR Text |
Show 1903.] METAMORPHOSES OF DECAPOD CRUSTACEANS. 29 C. fasciatus cannot be separated generically as a larva from the other species named. A characteristic of the group to which it belongs is the paired arrangement of the abdominal spines. It is true that they are practically absent in C. trispinosus, but their occasional appearance on one segment shows that their loss is a recent one. Classifying therefore on the basis of the larval form, we must merge the genus Cheraphilus into uEgeon, leaving the genus Pontophilus to include P. spinosus and P. norvegicus. This arrangement seems to fit the adults as well as it does the larva?. In the first place, Ortmann seems to have made an error in including all the above-mentioned forms in a single genus Pontophilus. H e gives as characters of this genus " Gills seven ; six pleurobranchs (i, k, I, m, n, o), and one rudimentary podo-branch h" ; but I have found the following gill-formula to hold for Cheraphilus trispinosus, C. nanus, and yEgeon fasciatus:- ! a. VI YIL ... Yin.... IX X. XI XII. ... XIII. ... Ep Ep Ep + b. 1 ... ... c. 1 1 1 1 1 = 6 + 2 or 3Ep In Ponto])hihts spinosus there are seven gills, there being a podobranch upon the second maxillipede. If this statement holds good for the remaining species, then the two genera Cheraphilus and jEgeon should be separated from Pontophilus. As regards their relation to one another, the only important differences between them seem to lie in the shape of the rostrum and the direction of the bend of the apices of the gills (Spence Bate). Such differences do not seem to m e of sufficient importance to allow of generic distinction, and I have therefore in the heading of this paper treated the two species dealt with as both belonging to the genus JEgeon Risso. Literatitre referred to in the foregoing paper. BATE, C. SPENCE.-Report on the Crustacea Macrura collected during the ' Challenger' Expedition. 1888. C L A U S , C.-Zur Kenntniss der Kreislaufsorgane der Schizopoden und Decapoden. Arb. Zool. Inst. Wien, v. pp. 271-318 (1884). C L A U S , C.-Zur Kenntniss cler Malacostracenlarven. Wiirzb. Naturwiss. Zeits. ii. pp. 23-46 (1861). |