REPORT ON THE CRUSTACEA MACRUHA. 27 



water through the branchial chamber cannot be very vigorous, and consequently in that 

 portion of the chamber that is most distant from the direct current, the circulal Lou will be 

 very inactive. In the central portion of the chamber the branchiae, instead of consisting 

 of cylindrical rods, are developed into thin foliaceous plates of considerable dimensions, 

 through the tenuity of the structure of which the blood is brought over a large surface 

 iuto contact with the aerating medium within the chamber. The circulatory channels 

 seen within the plates demonstrate the organs to be of a complex structure, and capable 

 of performing a function of a less simple kind — namely, of extracting the oxygen from 

 water that has been stored for a long period where it has not been affected by the 

 atmosphere. 



I am aware that this is mere speculative reasoning from the appearance and condition 

 of the organs, and that we must know more about the habits and mode of existence of 

 the animal before we can determine with certainty what separate duties or functions 

 these two varieties of branchial organs effect. 



That neither of them can be the depauperised form of the other I feel assured, 

 inasmuch as the variation in the several parts of the animal exhibits no depreciation of 

 structure. 



The various organs may be abnormal in form, but they are evidently well adapted 

 for their purpose ; the mandibles are strong, and the synaphipod, which is required 

 for sweeping food within reach of the incisive margin of the mandible, is a very 

 powerful and efficient appendage. The stomach is a large and capacious organ, 

 complicated in its structure, and adapted for the comminution of substances into a form 

 that adapts them for assimilation. 



The points of interest which this Crustacean possesses have induced me to give a 

 detailed description of the several parts of its structure. Although the animal is not 

 new, it has never been fully described ; and the only figures, so far as I am aware, are 

 a small one in Desmarest's Consid. des Crustaces, another in Leach's Miscellany, and 

 in Cuvier's Atlas to the Regne Animale. 



The animal is interesting, and not only supplies a link connecting the Macrurous 

 with the Anomurous Crustacea, but also shows that the trichobranchiate structure is 

 intimately associated with the phyllobranchiate form, that the one is only a modification 

 of the other in adaptation to varied conditions. 



Family Callianassid^e. 



The several genera of this famdy are conveniently determined by the gradually 

 increasing size of the somites of the pleon, together with the great breadth of the fobaceous 

 plates of the rhipidura, and by the asymmetrical character of the first pair of pereiopoda, 

 which generally have a tendency for the right to be larger, deeper, and less perfectly 



