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Show 1886.] DURING THE VOYAGE OF H.M.S. 'CHALLENGER.' 113 is represented by a single specimen, which was dredged in the Pacific Ocean at Station 218, in 1070 fathoms of water. It is a large Isopod, measuring upwards of two inches in length, and agrees in most particulars with such genera of the Cymothoidse as Mga ; but at the same time it presents certain remarkable peculiarities analogous to those exhibited by the aberrant genus Bathynomus lately described by Prof. A. Milne-Edwards from deep water in the North Atlantic. M. Milne-Edwards's preliminary account of Bathynomus was communicated to the French Academy1, and a translation of his note has appeared in the Ann. & Mag. of Nat. Hist.2 Apart from its huge size, the most remarkable feature in the organization of Bathynomus is the great development of branchial organs : " it appears," says M . Milne-Edwards, " that the respiratory apparatus of an ordinary Isopod is insufficient to supply the physiological needs of Bathynomus, and that the development of special organs of a greater functional power has been rendered necessary. The abdominal limbs, which usually in this group constitute the sole branchial apparatus, in Bathynomus only serve the function of a covering to the gills which lie beneath them." The gills of this Crustacean are in fact represented by a series of complicated branched outgrowths of the body-wall in the ventral region of the abdomen. The same end is attained by the Crustacean, which forms the subject of the present remarks, in a different manner. Instead of a development of accessory respiratory organs, Anuropus (as I may term the genus from its chief structural peculiarity) exemplifies one extreme of the Isopodan type, in that all the abdominal appendages are converted into respiratory organs ; the increase of respiratory surface is thus attained by an exaggeration of a.structural character, which is common to all the members of the family, and which indeed is an important basis of distinction from other families of Crustacea. In all the members of this group more or fewer of the abdominal limbs are soft foliaceous appendages, which permit of an easy exchange of gases between the contained blood and the sea-water. There is no instance, however, among the Isopoda in which all the abdominal appendages are similar, functioning as respiratory organs, except in this deep-sea genus Anuropus. Bathynomus, as regards the uropoda, is quite a typical Isopod ; these appendages form a pair of swimming-feet as they do in the other Cymothoidae 3. The modifications of the terminal pair of abdominal appendages or uropoda serve to divide the Isopoda into natural families, which prove to be allied in other particulars ; and some stress, therefore, from a classificatory point of view, should perhaps be laid upon the fact of their modification in Anuropus, though it is always open to question how far a purely adaptive character is of value. Since the present genus agrees with the Cymothoidae in the general form of the body, in the number of free abdominal segments, and in fact in all essentials, it would perhaps be hardly permissible to remove it 1 ' Comptes Eendus,' Jan. 1879. 2 Ann. & Mag. Nat. Hist. 1879 (vol. iii.), p. 241.. 3 There is a figure of Bathynomus in an interesting wort recently published by M . Filhol and entitled ' La vie au fond des mers,' Paris, 1885. P R O C ZOOL. Soc-1886, No. VIII. 8 |
