26 REPORT — 1855. 



free ; not being protected b)^ the carapace, it ceases to possess that resem- 

 blance to an internal skeleton, vvliich it receives in the higher types from its 

 peculiar relation to the monstrously developed cephalic rings. 



In the Anqihipoda the upper portion or shell of the cephalic ring is 

 constructed as in the higher types, that is, it is formed of the antennal 

 and mandibular segments, each reduced to almost its minimum of im- 

 portance. 



In TaUtrusaxxA Gammarus, but most distinct in consequence of the larger 

 size in the former, a suture, which most certainly homologizes with the so- 

 called cervical or epimeral suture of Macroura, is visible, and shows that the 

 mandibular ring perfects its inferior arch: this forms the epistome of the 

 frontal aspect of the head. 



The line of demarcation or suture which separates this segment from the 

 anterior, traverses the lateral walls of the head, parallel with, and but a short 

 distance above, the mandibles, after passing which it rises toward the upper 

 surface, but loses itself in the posterior margin about half-way from the top*. 

 In this respect it bears some analogy to the manner in which it is lost in 

 Srachyura, but only in appearance, for there it was the result of a large 

 development of the anterior segments ; here both are equally unimportant. 

 In point of fact, the connexion of the Amphipoda is mucli nearer to the Ma- 

 croura; and if a perpendicular line of incision were made to cut away the 

 carapace of Astacus ^ust in front of the cervical suture where it exists on the 

 top of the carapace, that is to remove the whole of the carapace posterior to 

 that line, and perfect each ring of the thorax, but for the pedunculated eyes, 

 the Astacus would be pronounced among the Amphipoda. 



The epistome (Plate XII. fig. 1 C) appears with little doubt to be the inferior 

 aspect of the mandibular ring (B), which is seen on the external lateral 

 surface of the head, and which can be identified from the fact of its carry- 

 ing the mandibles. This relation of the epistome to the mandibular segment 

 is not admitted by Mr. Dana, who rather, from analogy with the higher 

 types, than by direct evidence of the subject before him, identifies the epi- 

 stome as belonging to the inferior (or external) antennal segments. 



We do not think that the evidence in the higher forms bears out this 

 assumed relation ; for whilst in the Brachyura the two antennal segments and 

 the mandibular, each through the arrangement of their sternal portions, unite 

 to form the antero-oral plate, we find that in the Macroura their relative 

 importance is not of equal extent. We think, that as the ophthalmic segment 

 is itself not developed to much importance in the Brachyura, and is altogether 

 lost in the Macroura, so we believe that the same process of annihilation of 

 parts continues, and that in the Amphipoda the only segment in which the 

 sternal portion is persistent is the mandibular. A thin partition of osseous 

 tissue, passing perpendicular in the median line between the antennae, less 

 important between the superior than the inferior, may possibly represent the 

 sternal part of each of the antennal segments respectively. 



The next three pairs of appendages succeeding the mandibles are borne 

 upon a piece which forms the infra- posterior portion of the head (Plate XII. 

 fig. 2 K), and is probably the sternal piece of the segments belonging to the 

 two maxillae and the maxilliped ; the dorsal portion of these segments 

 appears to form an arch within the cavity of the head, as given in Plate XII. 

 fig. 3, and offers a support to the stomach as well as points of attachment 

 for muscles. 



In attributing to this internal structure the high relative importance as the 



* This suture, though recognized, was scarcely appreciated by us until we had read 

 Dana's work. 



