XVI INTRODUCTION. 



sequently always formed of two layers, soldered, as it 

 were, together." * 



Of the various segments composing the three principal 

 portions of the body, the head, the thorax, and the abdo- 

 men, some are found always to support similar, or rather 

 identical, organs. Thus the first cephalic segment or ring 

 invariably bears the peduncle of the eyes, and the second, 

 or antennary, as constantly supports a pair of the antennae. 

 Of those which follow, there are the most extraordinary 

 and unlooked-for modifications in the different groups ; 

 and no one who has only formed a theoretical notion of 

 these parts could recognise in the simple piece of which 

 the whole cephalic region is composed in the Edriophtlialma, 

 or in the carapace or shell of the brachyurous Decapoda, as 

 in the common crab for instance, the mere combination of 

 two or more of the cephalic segments which in other forms 

 are found to be distinct. For a full and clear account of 

 all these modifications, the reader is referred to the admi- 

 rable work of Dr. Milne Edwards, so often quoted and 

 referred to. 



This author has, with great propriety, considered the 

 genus Squilla as offering the form in which the different 

 segments before enumerated are most distinctly exhibited ; 

 but even in this form there are some which are, as it were, 

 soldered together, and the normal number is consequently 

 not to be traced. The first cephalic segment, which, as 

 before observed, is invariably destined to support the 

 ocular peduncles, and is therefore termed the ophthalmic 

 segment, is here quite distinct from the second, which is 

 also very distinctly articulated with the third ; the latter 

 is, however, confounded with the next, and the following 

 ones are only to be distinguished by dissection. -f- But the 



* Edw. Hist. Nat. dcs Crust, i. p. 18. + lb. p. 15. 



