OCR Text |
Show 1899.] CRUSTACEANS FROM LAKE TANGANYIKA. 707 respectively, while the dactylus of the fifth pair was half as long again and had a row of 39 spines. In the female, the first pair of pleopods (Pl. X L . fig. 14) have the endopod rather slender, pointed, and more than half the length of the exopod. In the male (Pl. X L . fig. 15), the endopod is a short ovate leaflet about one-quarter the length of the exopod. In nearly all the specimens of both sexes the first pair of pleopods are turned forward, with the exopod lying above and external to the bases of the posterior peraeopods. According to F. Midler (Kosmos, ix. 1881, p. 121), this is the position taken by these appendages in the living Atyoida, and he states that they serve to protect the entrance to the branchial chamber, the fringe of marginal setae acting as a sieve to exclude mud, &c. In the second pleopods of the male (Pl. X L . figs. 17, 17a), the appendix masculina is a little shorter than the appendix interna, and bears a number of stout spines. The telson (Pl. X L . fig. 18), is about as long as the inuer plates of the uropods, with straight sides, tapering to the obtusely pointed tip which bears four spines, two short external and two longer internal, between which latter spring three plumose setae. On the dorsal surface of the telson are two pairs of spinules. In G. wyckii the tip of the telson bears eight spines, and the dorsal surface three pairs of spinules. The gills are four in number on either side, three pleurobranchs, corresponding to the second, third, and fourth peraeopods, and one which I believe to be a pleurobranch (though it is difficult to determine the precise point of insertion) above the first peraeopod. There are no epipods on the maxillipeds or peraeopods, unless we regard as a rudimentary epipod the small papilla at the base of the third maxilliped described above. In tabular form the arrangement is:- Pleurobranchias... Podobranchiae ... Arth r obran chiae... Total mxp.2 msp.3 per.1 1 per.2 1 per.3 1 per.4 I per.5 4 4 The statements of various authors as to the branchial formulae of the genera of Atyidce are somewhat conflicting, but all agree in giving a larger number of gills and a complete series of epipods as far as the fourth peraeopods.* * F. Muller states (I. c. p. 121) that in Atyoida potimirim the last two pairs of legs are without epipods. |