254 THE CRUSTACEA 



lateral walls of the thoracic somites (pleurobranchiae), very rarely- 

 absent ; young rarely hatched in nauplius-stage. 



Historical. — The great majority of the larger and more familiar 

 Crustacea belong to the Decapoda, and this Order received far more 

 attention from the older naturalists than any of the others. A 

 considerable number of species are mentioned by Aristotle, who 

 describes various points of their anatomy and habits with accuracy, 

 and sometimes with surprising detail. A long series of purely 

 descriptive writers who have added to the number of known forms 

 without contributing much to a scientific knowledge of them begins 

 with Belon and Rondelet in the sixteenth century, and perhaps 

 does not altogether come to an end with Herbst's Naturgeschkhte 

 der Krabhen unci Krebse (1782-1804). Among the most noteworthy 

 of early contributions to anatomy are Swammerdam's memoir on 

 the Hermit-Crab (1737), and that of Roesel von Rosenhof on the 

 Crayfish (1755). Reaumur's observations on the phenomena of 

 ecdysis and the regeneration of lost parts in the Crayfish (1712- 

 1718) have become classical. The foundations of classification were 

 laid by J. C. Fabricius (1793), who divided the Linnean genus Cancer 

 into a large number of genera, the majority of which are still 

 recognised. Latreille, to whom the name of the Order is due (1802), 

 also began its subdivision into sub-orders and families. In this 

 more than in any other group of Crustacea the works of H. Milne- 

 Edwards, and especially his Histoire Naturellc cles Crustacds, may be 

 taken as marking the beginning of the modern period, and his 

 classification of the Decapoda has been that most generally accepted 

 until very recently. Almost contemporaneous with Milne-Edwards's 

 great work, and often surpassing it in morphological detail and 

 systematic insight, was de Haan's volume on the Crustacea of Japan 

 (1833-1849). The first important departure from the general plan 

 of classification laid down by these authors was made by Boas in 

 1880, and his system has been further elaborated by Ortmann and 

 by Borradaile. J. Vaughan Thompson's discovery of the larval 

 metamorphosis of Decapoda (1828-1831), confirming the earlier 

 observations of Slabber and Cavolini in the eighteenth century, 

 gave rise to a curious controversy in which Westwood and others 

 denied the possibility of such a metamorjDhosis, basing their argu- 

 ments chiefly on Rathke's memoir on the development of the 

 Crayfish (1829). F. Midler in 1863 made the highly important 

 discovery that Penaeus is hatched from the egg in the form of a 

 nauplius, and the clue thus given to the interpretation of the other 

 larval stages was followed up especially by Claus. The development 

 of deep-sea exploration within the last thirty years has resulted in 

 the discovery of a large number of important new types of Decapoda, 

 which have been described by Spence Bate, Miers, Henderson, A. 

 Milne-Edwards, Bouvier, Faxon, Alcock, and others. The numerous 



