ON OUR PRESENT KNOWLEDGE OF THE CRUSTACEA. 47 



[pi. 1. fig. 3 of his work]. The former results from the assemblage of four 

 pieces more or less intimately connected together, and arranged in pairs on 

 each side of the median line. The central pieces are called by the name of 

 the tergum, and the lateral are called the Jiancs or epimeral pieces. The 

 inferior arc is composed of the same number of pieces. The two median 

 pieces unite to form the sternum ; and the latter are known by the name of 

 tlao episternum, by reason of their analogy with those that M. Audouin has 

 designated by the same name among insects. They are united always at the 

 sternum ; but there generally exists, between the inferior arc and the epimera 

 situated above, a wide space destined for the articulation of the corresponding 

 member." 



" "We know of no example," he continues, " of a ring where we are able 

 to distinguish at the same time all the pieces that we desire to enumerate. 

 Sometimes there is an absence of some of the pieces from the place they 

 should occupy, and sometimes they are very intimately soldered together, so 

 that we cannot see even a trace of separation ; but in studying each of them 

 separately, where it is most distinct, we shall be able to form a clear idea, and 

 recognize its character in spite of its union with its neighbouring pieces. 

 Moreover, although this analysis of the ring may not be always practicable, 

 it is not the least true that it facilitates much the study of the exterior ske- 

 leton of articulated animals, and that it wlU permit us often to establish 

 analogies where there would first appear to exist the greatest difference." 



" To terminate the enumeration of the constituent parts of the tegumentary 

 rings of the Crustacea, there only remains for us to speak of the plates that 

 we often see elevated from the internal siu'face aiid arrange themselves into 

 cells and canals. These processes are always developed at the points of union 

 of two rings or of two neighbouring pieces of the same segment ; and this 

 disposition has obtained for them the name of apodemes (from M. Audouin). 

 They are the result of a fold of the integumentary membrane which penetrates 

 more or less deeply between the oi'gans, and which is strengthened with cal- 

 careous matter like the rest of the structure, and "are always formed of two 

 thin plates soldered together." 



These views have long been accepted as the acknowledged theory. Isor 

 am I aware that any one (except the authors above quoted) has attempted 

 upon original investigation to analyze the evidence upon which M. Milne- 

 Edwai'ds has formed his theory. 



That the author of this lieport has long held views not consistent with 

 M. MUne-Edwards's theory, is known to those carcinologists who have read 

 his Eeport on the British Edriophthalmia, which was communicated to this 

 Association and piiblished in its Transactions for 1855, wherein he trusts 

 that he clearly demonstrated that the pieces to which M. Milne-Edwards 

 gave the name of epimera, and selected by him as typical of his theory, 

 were parts attached to the legs, and not pieces of the dorsal arc of the 

 crustacean somite. 



He is moreover desirous in this Report to show : — that the epimera, as sec- 

 tional pieces in a theoretical construction of a somite, cannot exist ; that 

 the so-called epimera are portions only of the integumentary structure of 

 the appendages of the animal, and that the apodema are formed out of 

 this structure, more or less thinned out by lateral pressure and internal 

 arrangement ; and that the head of the lower ty^jes and carapace of the 

 higher are homologically the same, the carapace being a monstrous deve- 

 lopment intended for the covering and protection of the more complicated 

 branchial appendages of the higlier types. 



