﻿OF THE ORDER MEROSTOMATA. 7 



These terms, used by Mr. C. Spence Bate 1 for the several joints of the limbs of 

 Crustacea, are merely abbreviations of Milne-Edwards's terms coxopodite, basipodite, &c, 

 coxognathite, basignathite, &c. The appendage may be a podoite, or a gnatltite ; but it 

 seems superfluous to repeat the term at every joint, especially in treating of the limbs 

 of Pterygoids, which are gnathopodites, or jaw-feet. 



The common Lobster and Prawn are very good typical forms ; but it is impossible 

 to offer in one view any general classification of the appendages of Crustacea which 

 is suitable to the whole group ; for the limbs, subservient to one purpose in the Decapod, 

 fulfil a totally different function in the Stomapod or Branchiopod ; or the segments them- 

 selves are so welded together in the one, and so separated in the other, as to require much 

 careful examination in order to discover their homologies. 



Indeed the only segment that may be said to be persistent, is that which supports 

 the mandibles, for the eyes may be wanting, and the antennae, though less liable to 

 changes than the remaining appendages, are nevertheless subject to very extraordinary 

 modifications, and have to perform functions equally various. Being essentially and 

 typically organs of touch, hearing, and perhaps of smell, in the highest Decapods, they 

 become converted into burrowing organs in the Scyttarida, organs of prehension in 

 the Merostomata, swimming organs and claspers for the male in the Cyclopoidea, and 

 organs of attachment in the Cirripedia. 



Not to multiply instances, we have presented to us in the Crustacea probably the 

 best zoological illustration of a class, constructed on a common type, retaining its 

 general characteristics, but capable of endless modification of its parts so as to suit the 

 extreme requirements of every separate species. 



And it is doubtless in some degree due to this plasticity of structure, enabling the 

 species to occupy such diverse positions, and to subsist upon such varied aliment, that 

 the class owes its preservation through the lapse of ages represented by the long series 

 of geological formations, from the very oldest Silurian strata to the present day. 



It is interesting to review the long and laborious methods by which the frag- 

 mentary fossil remains of the ancient order of the Eurypterida have, by the labours of 

 Agassiz, M'Coy/ Hall, Huxley, Salter, and others, been made, after frequent read- 

 justment, not only to fit together correctly, so as to furnish us with a notion of their 

 living forms, but also to take their appropriate places in the class and order to which 

 they belong. 2 



As I shall have frequent occasion to refer to the published labours of these gentlemen 

 in the course of this Monograph, I will not further allude to them here. 



The additional materials which have accumulated during the past eight years suffi- 



1 ' Phil. Trans.,' 1858, p. 604. 



2 See Messrs. Huxley and Salter's 'Monograph,' already quoted, pp. 1 — 11. See also article by the 

 writer "On the Seraphim and its Allies," in the ' Intellectual Observer,' 18G3, vol. iv, p. 229. 



