38 Mr. C. Spence Bate on the Homologies of the Carapace 



out, that the small central patch bears the two antennae, that 

 the external angles of the carapace which pass in front and 

 surround it posteriorly, carry the mandibles, demonstrates the 

 relation of one portion of the carapace to the other, and that the 

 line of separation round the autennal centre homologizes with the 

 cervical suture of the Macroura. 



The constant position of this suture in all Crustacea when 

 present is the same, and forms a line of demarcation between 

 the third and the fourth rings, and therefore visible in its posi- 

 tion external to the inferior antennae ; and M. Milne-Edwards is 

 most assuredly wrong when he attributes the depressions on the 

 carapace, which terminate in the central notch of the orbits of 

 the JBrachyura, to be the representatives of the cervical suture of 

 the Macroura. 



If we wish to jiidge of its position in the Brachyura, it is but 

 just that we should make a careful investigation of the structure 

 of the animal in its immature condition. In the so-called pupa 

 stage of the Crab, we find that the inferior antennse are attached 

 to the extreme horns of the carapace (PI. I. fig. 7), but these 

 horns are folded beneath the animal ; it is this reflexion which 

 afterwards forms the orbit in which the eye is lodged. 



The position of the antenna, anchylosed as it is with the der- 

 mal skeleton in all the Brachyura, still holds the same, therefore 

 by inversion the cervical suture must be inferiorly inside, but 

 still within the limits of the carapace ; such a suture is plainly 

 demonstrable in most of the Brachyura (PI. I. fig. 10, and 

 PI. II. fig. 1), and separates the inferior antennal (a) from the 

 mandibular ring {b) ; it extends posteriorly to the extreme 

 limits of the carapace, forming as it were two side pieces, the 

 epimerals of M. Milne-Edwards : this line unquestionably homo- 

 logizes with the cervical suture of the Macroura. 



If we turn our attention to the development of the nervous 

 system in these various animals, we shall find that centralization 

 decreases in an inverse ratio with tlie development of that por- 

 tion of the carapace which is posterior to the cervical suture, and 

 vice versa, that centralization is most perfect when that portion 

 of the cara])ace which is anterior to the cervical suture is largest. 



In the Brachyura the nervous ganglia arc in the highest de- 

 gree consoHdated, and in the Spider Crabs the most perfect cen- 

 tralization exists; there we shall find that the cervical suture, 

 the line of union between the inferior antennal and mandibular 

 rings, is so lateral, that the two side pieces of the incomplete 

 mandibular ring are reduced to much less importance than is to 

 be found in any other tribe of the whole class. In the genus 

 Cancer, &c. the line of union remains but partially anchylosed, 

 and splits when the animal throws off its exuviae. 



