46 REPORT — 1875. 



as to the homology of the carapace, but denying the existence of epimera in the 

 theory of the somite, and corroborating the assertion of Dana that the antennal 

 segment constitutes the anterior and upper portion, and the mandibular seg- 

 ment the posterior and lower portion of the carapace in the Macrura and Era- 

 chyura ; and affirmed that the suture which traverses the lower surface forms 

 a line of demarcation between the third and fourth somites ; it homologizes 

 with the cervical suture in the Macrura, as also with that which traverses the 

 dorsal surface of the cephalon in several genera of Trilobites (PL I. fig. 5). 



If we wish to judge of the relation of these parts in the several forms of 

 Crustacea, we must make a careful investigation during the immatiu'e stages 

 of the animal. 



In the Megalopa stage the inferior antennae are attached to the anterior 

 external horns of the carapace ; these horns are folded beneath the animal, 

 and it is this inflection that forms the orbit in which the eye is lodged. 

 Through this inversion, consequent upon the monstrous development of the 

 hepatic region, this suture lies upon the inferior surface of the carapace in 

 Erachyurous Crustacea, extending posteriorly to the extreme limits of the 

 carapace. 



The author concluded his paper by saying, " But we have seen in the de- 

 scending scale of nervous force the rings which carrj' the organs of conscious- 

 ness degenerate in importance, and yield to a corresponding development of 

 the mandibular ring : this law appears to be in force in the Amphipoda, the 

 lowest type of the Macrura form, in which I am inclined to believe that the 

 mandibular ring represents the whole of the upper portion of the cephalic ar- 

 ticulation — the anterior three being so diminished in importance, that they are 

 to be found only in the perpendicular wall of the head, or perhaps represented 

 by their appendages only" (Ann. Nat. Hist., July 1855). 



It would scarcely perhaps be necessary to enter further into the evidence 

 that supports the homological relations of the carapace, had not Professor 

 Huxley, in his Hunterian Lectures at the Eoyal College of Surgeons, expressed 

 an opinion opposed to the above statements. 



In his twelfth lecture Prof. Huxley says : — " In aU the Brachyura and 

 ordinary Macrura it appears to me to be obvious that the carapace is con- 

 tinuoiis with, and part of, all the somites of the cephalothorax- — that it is 

 composed, in fact, of their connate terga, the branchiostegite being nothing 

 more than their connate and highly developed pleura ; the cervical suture, 

 placed immediately behind the attachment of the mandibular muscles and in 

 front of the heart, corresponds in these respects precisely with the posterior 

 boundary of the head of a SquiUa and of a Branchiopod, or of an Edrioph- 

 thalmian. The cephalic arc roofs over the stomach, as does the tergal region 

 of the head in these last-named Crustacea. Anatomically, then, it seems to 

 be demonstrable that the scapular arc of the carapace in the ordinary Podoph- 

 thalmia is the equivalent of the terga of the thorax, that the cephalic arc 

 is the homologue of the terga of the head, and that the carapace is formed 

 by aU the cephalothoracic somites." 



Before the Reporter can proceed with any fresh evidence to support the 

 argument demonstrative of the homological character of the carapace, it is 

 desirable that a clear idea should be given of the theory of a somite or 

 segment as it exists in Crustacea. 



Prof. Milne-Edwards, in his ' Histoire des Crustaces,' vol. i. p. 16, says : — ■ 

 " Each of the rings of the skeleton appears to be composed of two lateral 

 moieties, resembling each other. We can distinguish moreover two arcs, 

 the one superior, the other inferior, as shown in the accompanying diagram 



