OCR Text |
Show 1868.] MR. c. SPENCE BATE O N N E W FRESHWATER PRAWNS. 367 with the outer margin of the second articulate margin ciliated, extremity of both plates round. Telson smooth, laterally sliglitly compressed; near the base on the central dorsal surface is a depression occupied by a small tuft of hair, and beyond the middle, on each side, are two short spines and a fasciculus of short hair. Hab. Tambo River. The near resemblance that these species bear to those of the genus Palcemon may induce some carcinologists to reconsider the propriety of making these species a genus by themselves. I am not aware that any structural distinction separates them from the genus Palcemon. There is, I think, however, in the enormous length of the second pair of pereiopoda, when compared with the same appendage in Palcemon, a strong prima facie evidence that a separate generic distinction would form a very natural classification. I had, I must admit, some doubts upon the question, and hesitated in my opinion until I found that others, though closely allied in general form, yet specifically distinct in character, enabled me to see that the peculiarly distinguishing features that separated the species of this genus from Palcemon were sufficiently constant to warrant the adoption of the new genus. The convenience of this arrangement may also be seen in the peculiar and distinct habitat of Macrobrachium, the whole of the speeies yet known being lacustrine or fluviatile. I have only seen one or two specimens of each species, and these are all males. The development of the chelopods is so great in length that it must be difficult, if not impossible, for the animal to reach its own mouth with them ; so that they can be of no use in feeding, for which purpose the first pair, being shorter, are more efficient. I believe it probable, but have not been informed, that in the females the chelopods are less monstrously developed. It is something very remarkable that these Prawns, all of them so very large, living in freshwater lakes and rivers, in localities so very distant from each other as Central America and Central India, should bear so near a resemblance. W e are not aware that the same rivers or lakes have any other species of Prawn ; and it would appear that the several species must have come from one common origin ; for even the position and number of the spines on the telson, as well as the fasciculus of hair in the small depression at the base of the same, are common to all the species. Whether or not there is anything remarkable in the form of their young or in the development of their larvae I know not. The freshwater Astaci differ from their marine congeners in producing the young in a more advanced stage of development; but this appears not to be a constant law in freshwater Crustacea. In a small freshwater Prawn from the rivers of the island of Mauritius, that has been sent to me by Dr. Power, the young undergoes a change of form similar to that of the marine species. I look upon the discovery of these edible Crustacea as being |