XVIII. On the development of Carcinus mænas

Abstract

The author, after noticing the history of the subject, and the opposition which the assertion, “that the Zoë of naturalists is the larva of a common crab,” received, traces the progress of the development of the animal from the Zæa to the adult, and endeavours to demonstrate, that from the youngest to the most perfect form, the changes are the result of no sudden transformation, but produced by a gradual series of alterations contemporary with every succeeding moult; that the Zæa is connected with the Megalopa , and the latter with the adult by many intermediate gradations, each in itself scarcely appreciable, and progressively approximating nearer and nearer the more perfect stages. The author asserts that the development is earliest and most complete anteriorly; that when first born, the seventh or posterior segment of the head, one or more of the posterior segments of the pereion (thorax), and the penultimate of the pleon (abdomen) are wanting in the brachyurous Decapods; but that this general law loses somewhat of its force in the descending scale of development; and as it becomes less persistent, the animal approximates in the larval condition nearer to the form of the adult type; while on the other hand, the same appears to be a constant law of the depreciation in adult forms, as exhibited in the more or less aberrant Amphipoda, such as Cyrtophium, Dulichia , &c.