545 



opposition which the assertion, " that the Zoe of naturalists is the 

 larva of a common crab," received, traces the progress of the deve- 

 lopment of the animal from the Zoea 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 Zoea 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 com- 

 plete 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 some- 

 what of its force in the descending scale of development ; and as it 

 becomes less persistent, the animal approximates in the larval con- 

 dition 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. The author likewise shows that 

 the appendages, which act the principal parts in the larvae, become 

 the secondary parts of the same organs in the perfect animal. For 

 instance, the lower antenna is represented in the larva by the com- 

 plementary appendage of the adult form ; the true antenna is de- 

 veloped from the base of the embryonic organ, which represents 

 the squamiform and spinous appendages, more or less constant in 

 the mucrourous Decapods, but lost iu the short-tailed genera, and 

 the organ itself is gradually increased with every successive moult. 

 This is true, more or less perfectly, of all the other appendages pre- 

 sent in the larvae of all Decapoda ; and no change of form, as under- 

 stood in the term metamorphosis as applied to insects, takes place in 

 the development of Carcinus. That the distance between the old 

 and young forms is the result of an exaggeration of parts in the 

 larva as compared with the relative proportion of the same in adult 

 animals, together with the absence of others, which are gradually 

 produced, and assume the permanent condition of the adult type. 



The author has observed the rudiments of the future legs shortly 



