ON 



THE KING CRAB 



{Limulus polyphemus. Late.). 



§ I. Introdnciion. — The living representatives of extinct groups of animals have always 

 had peculiar attractions for my scalpel, especially when the lost group was large and the 

 dissectible representative rare and exceptional in character. Such, e. g., was the EZivi*, 

 sole survivor of the race of Moas ; such the Protapterus or Lepldosirenf, the living repre- 

 sentative of extinct notochordal, protocercal, cycloganoid fishes of palaeozoic seas ; such the 

 Nautilus, in like relation to the extinct fabricators of chambered and siphonated shells | ; 

 such also were Terehvatula, Lingula, and Dlscina §, as representatives of the Brachiopoda, 

 and Euplectella, the surviving type in oceanic depths of the fenestrate Venti-icidites. 



"With reference to the singular and interesting palaeozoic Crustacea known chiefly, if 

 not exclusively, in 1840, as ' Trilobites,' I was for a time uncertain whether to take the 

 rare Isopod SeroUs, of which a specimen was procured for me for that purpose by my 

 friend Charles Stokes, Esq., F.R.S., the discoverer of the 'labium' or lip-plate in Trilo- 

 bites {Asaphus platycep]ialus)\\, or to look for their grade and plan of internal structure 

 in Lhmihis. 



The authority of W. Sharpe Macleay, after the appearance of his famous "Horse 

 Entomologicae," weighed about that time heavily upon us. All who had studied the 

 Trilobites up to 1843 were of opinion that they were malacostracous. Audouin led the 

 way by affining them to the Isopoda ^ ; and Macleay, in an Appendix to Murchison's 

 great work on the Silurian strata, assigned to Trilobites a position as a distinct Order 

 between the Isopoda and Aspidophora, basing his views on the trilobed character of the 

 segments in SeroUs and Bopyrus, and the character of the eyes in Cymothoa, which " were 

 large, sessile, and compound, as in Trilobites. Moreover Cymothoa and other Isopods," 

 he remarked, " rolled themselves into a ball," as Trilobites have been found to do before 

 they perished. 



The first general fact or view which influenced my choice in this matter was the 

 character of the Malacostraca, founded on the number of body-segments, — seven for 



* " On the Anatomy of the Apteryx avstralis," Trans, of Zool. Soc. vols. ii. & iii. (1838). 



t Trans. liun. Soc. vol. xviii. 1839. 



+ ' Memoir on the Pearly Nautilus {Nautilus pompilius),' 1832. 



§ " On the Anatomy of the Brachiopoda of Cuvier," Trans. Zool. Soc. vol. i. (1835); also ' On the ^inatomy of 

 Terehratula and Lingula,' ilonograi^h, published by the Palteontographieal Society in vol. for 1854. (The subjects 

 for the anatomy of Discina, Sow., were referred to the genus Orbicula.) 



II Trans. Geol. Soc. Lond., N. S. vol. i. pi. 27. 



1[ " Eechcrches sur les Eapports Naturols qui existent entre les Trilobites et les Animaux Articules," Annales des 

 Sciences Physiques de Bruxelles, torn. viii. (1821). 



