Biological Notes

Abstract

CLASSIFICATION OF DECAPOD CRUSTACEANS.—In this well-defined group, the position of the anomurous forms (hermit-crabs, &c.) has often been the subject of doubt. The special adaptations of some genera for particular modes of life have caused them to be thrown together; and no doubt they agree in possessing neither the powerful abdomen of the lobsters, nor the very much aborted one of the crabs. Yet the anomurous forms include markedly contrasted groups. The family Hippidae, with its lobster-like cephalothorax and firm abdomen, differs greatly in aspect from the hermit-crabs. Hippa talpoida, a small species found along the whole eastern coast of the United States, inhabits sandy beaches exposed to the waves, at a zone very near low-water mark. It has a smooth oval form, and short and stout thoracic legs (second, third, and fourth pairs), enabling it to burrow backwards in the sand with marvellous rapidity. In life the antenna: are peculiarly crossed, with the flagella curved round the mouth so that the setas, with which they are densely covered, all project inwards, and the function of the antennæ appears to consist chiefly in the removal of all parasitic growths or foreign bodies from the anterior parts of the body. The appendages of the mouth are not adapted for prehension or mastication, and the alimentary canal is found loaded with fine sand. The thoracic appendages have neither external nor superior elements (exopodites, epipodites); white the office of protecting and cleaning the gills is discharged by the small limbs corresponding to the fifth pair of ambulatory legs in lobsters, which are curved upwards and hidden beneath the carapace. The development of this form has been recently carefully described by Mr. Sidney Smith, of Yale College, in the Transactions of the Connecticut Academy, vol. iii. p. 311. They pass through larval stages very analogous to the zoëa stages of crabs, only being destitute of a large dorsal spine; and they then assume a form like the brachyuran megalops, with large eyes, and powerful abdominal swimming legs. But in this condition they buried themselves in sand with great alacrity. Thus it is determined that the embryonic development of Hippa, as well as of Albunea, studied by Claus, agrees much more closely with that of crabs proper than with hermit crabs or lobsters; and this publication by Mr. Smith furnishes an important addition to the evidence favouring the view that the Anomura are a heterogeneous group made up of specialised families of Brachyura and Macrura.