Common Crayifish. 105 



For the points of difference between Astacus and Homarus, see 

 Milne-Edwards^ Histoire des Crustaces, vol. \., pp. 329-333. 

 For the American subgenus Cambarus, to which the species 

 inhabiting the mammoth cave in Kentucky belongs^ see Dana, 

 Crustacea, United States Exploring Expedition, J ^52, p. 522. 



For the nervous systems of the common lobster, see Swan, Comp. 

 Anat. Nerv. System, pi. iii. and iv., where the endophragmal 

 arch formed by the mandibulo-maxillary apodema, and inter- 

 posed in the natural position of the parts between the stomach 

 and the cords of the nerve collar, and the first sub-oesophageal 

 ganglion is well seen. See also Newport, Phil. Trans. 1834, 

 pi. xvii., fig. 40. 



For an account of the development of the common lobster, Homarus 

 Vulgaris, see Rathke, Wiegman^s Archiv. fiir Naturgesehichte, 

 1840, p. 241, translated in Annals and Magazine of Natural 

 History for 1841, vol. vi., p. 263, where the embryo on the 

 point of hatching is shown to differ from the adult mainly by 

 the presence on the ambulatory legs of an exopodite, such as 

 the two posterior maxillipeds retain in the adult Decapod, and 

 the ambulatory legs themselves in Mysis and Squilla. 



For the correlation of large size of ova with the completion of 

 development before hatching, see Bergmann and Leuckart, 

 Vergleichende Physiologic und Anatomic, p. 647 ; and for an 

 exemplification of this law, in the instance of Mysis, where 

 the eyes are few (fifty) in number and large, and where the 

 development is direct and without metamorphosis, as com- 

 pared with such cases as those of the Palinuri and Carcini, 

 where the eggs may be from one to three hundred thousand, 

 and are of small size, and where the well-known metamor- 

 phoses with forms known as *Zoea^ and 'Megalopa^ are 

 gone through, see Van Beneden, Recherches sur les Crustaces, 

 pp. 52, ^'>^^ 57. The fresh-water congeners of marine species 

 which go through metamorphoses, are very frequently ameta- 

 bolous in the sub-kingdoms of Mollusca and Vermes as well 

 as in that of Arthropoda. In the instances here under com- 

 parison, of the fresh-water Astacus and the marine Homarus, 

 it should be borne in mind that the ova of the former animal 

 are larger than those of the latter, though the adult Crayfish 

 rarely attains one-third of the size of the lobster. 



