512 CRUSTACEA MALACOSTRACA. 



eyes are stalked and compound. The first, second and third thoracic 

 appendages are modified as maxillipeds, leaving five pairs of trunk 

 legs, and there may be as many as four epipodial branchial rudiments 

 corresponding to each of the thoracic legs. 



The cephalo-thoracic shield or carapace attains its fullest 

 development among the Decapods, involving in many cases all 

 the thoracic segments, though in some groups the last segment 

 (Astacus) in others the two posterior (Brachyura) remain free. 

 The side of the carapace is formed by the pleural fold or branchi- 

 ostegite, which ends in a free margin over the bases of the limbs 

 and shelters the gills which lie between it and the body wall. 



In the Macrura Natantia and Reptantia the cephalo-thorax 

 is of an oval shape, ending in front in a pointed rostrum, the 

 branchial cavities are lateral, and the abdomen is well developed, 

 being as long as or longer than the carapace, and ends in a telson, 

 which with the terminal appendages forms a powerful caudal fin. 



In the Brachyura, on the other hand, the carapace is gener- 

 ally broader than it is long, the thoracic limbs are set wide 

 apart, and the short abdomen does not end in a caudal fin but 

 is bent forward and applied to the faces of the broad thoracic 

 sterna (Fig. 316). Owing to the great expansion of the cephalo- 

 thorax the branchial cavities lie rather beneath than at the sides 

 of the thorax and the branchiostegite, especially in the anterior 

 region, projects inwards forming the floor of the cavity (Fig. 316). 



In the Anomura we meet with a variety of stages intermediate 

 between the Macrura and the Brachyura, in these respects. 



In the larger Decapods the skeleton is firmly calcified. 



The so-called " cervical groove " of Astacus and many of the larger 

 Macrura is regarded by Huxley,* and some later writers, as the limit 



excreteur d. Crustaces Decapodes, Arch. Zool. exp., 1892. A. Ortmann, 

 Das Syst. d. Dekapodenkrebse, Zool. Jahrb. ix, 1897. See also Bronn's 

 Theirreich, Crustacea Malacostraca, Leipzig. T. R. B. Stebbing, A History 

 of the Crustacea. London, 1893 (Int. Scientific Series). A. Alcock, Materials 

 for a Carcinological Fauna of India, Journ. Asiat. Soc. Bengal, Calcutta, 

 1895-1900. Id. Desc. Catalogue of Indian Deep-Sea Crustacea Decapoda and 

 Anomala in the Indian Museum, Calcutta, 4to, 1901. W. T. Caiman, On 

 the Classification of the Crustacea Malacostraca, Ann. and Mag. of N. H., 

 Ser. 7, vol. xiii, 1904. N. K. Koltzoff, Studien lib. d. Gestalt d. Zelle. 

 I. Unters. lib. d. Spermien dt Decapoden, Arch. f. mik. Anat. Bd. Ixvii 

 (1906), p. 364. L. A. Borradaile, On the Classification of the Decapod 

 Crustaceans, Ann. and Mag. of N. H., Ser. 7, vol. xix (1907), p. 457. Id. 

 Articles on Crustacea in J. S. Gardiner's Fauna and Geography of the 

 Maldive and Laccadive Archipelagoes, Cambridge. 

 * Crayfish, p. 19. 



