ON OUtt PRESENT KNOWLEDGE OF THE CRUSTACEA. 45 



congeners the two anterior somites are distinctly separated from each other and 

 the third. In Palinurus the first is distinct from the second; but in the 

 greater portion of Brachynrous and Macrurous Crustacea the three first 

 somites, and perhaps the fourth, are strongly soldered into one piece. 



This piece in most Crustacea, but more conspicuously so in the more con- 

 densed forms, is developed to a greater or less extent, and is recognized under 

 the name of the carapace or shield. 



In the lower forms, such as the Amphipoda and Isopoda, it is developed 

 sufficiently to cover only the four succeeding somites ; while in the higher 

 forms, such as the Brachyura, it is developed so as to protect the whole of the 

 animal. 



The carapace varies very much in shape, both in width and length, and 

 generally covers the whole of the somites of the pereion ; but not universally 

 so. In the Anomura several genera have the posterior somite of the pereion 

 exposed ; in the Diastylidse there are three or four somites not covered, and 

 in the Edriophthalmic Crustacea all seven are unprotected and developed into 

 perfect somites. 



It is one of the earliest features present in the development of the embryo, 

 and is distinctly defined in the Nauplins form. Even in this early stage 

 of development, as in later existence, the form of the carapace varies 

 considerably, and is an easy mark of distinction between genera. It is 

 desirable as well as important, in an anatomical point of view, that a clear 

 idea should be obtained of the homological relation of this large and con- 

 spicuous portion of the highly developed cnistacean. This can be done only 

 after an examination and comparison of a large number of various forms 

 and types of animals, as well as a close investigation and study of the parts 

 during their progressive development. 



Milne-Edwards, as far back as 1834, arrived at the conclusion that the 

 carapace in the higher types of Crustacea is " the result of an excessive 

 development of the superior arch of the cephalic antenno-maxillary seg- 

 ment. . . . But (Hist, des Crust, vol. i. p. 2Q) among certain Stomapods, 

 such as Squilla, the head is divided into many distinct segments ; the first 

 two, the ophthalmic and antennular rings, are movable and little developed. 

 The third and fourth rings are, on the contrary, very large and compose 

 between them a single segment that we call the antenno-maxillary. The 

 carapace occupies the dorsal portion of the tron^on formed by this union, 

 and is prolonged above the six following rings." 



" In studying (I. c. p. 28) the carapace as a whole as well as in its parts, 

 we must examine into the rules of the normal organization of Crustacea, not 

 only in the later, more or less, remarkable modification, but also the very 

 curious structure of certain Eutomostraca, where all the animal is enclosed in 

 a kind of bivalve shell." 



These views receive general support from Mr. Dana, who, however, takes 

 exception to the assertion that the ventral piece of the carapace is formed 

 out of what M. Milne-Edwards calls the epimera {J. c. p. 32), but contends 

 that they " are in fact the posterior extension of the mandibular segment ; " 

 and he continues, " excepting that we consider what is here called epimeral, 

 the mandibular segment, we agree with Milne-Edwards, for the most part, in 

 the above-mentioned deduction ; so that while the mandibular segment is 

 confined to the ventral pieces of the Brachyural carapax, it constitutes its 

 posterior half in Macrura." 



In 18.55 the author of this Report communicated to the 'Annals of Natural 

 History ' a memoir on this subject, supporting the opinion of Jtlilue-Edwards 



