ON THE BRITISH EDBIOPHTHALMA. ~ 27 



homologue of thft dorsal portion of the segments, of which it is a part, we 

 think we are justified from a careful observation of its relation to sur- 

 rounding parts ; and it should always be borne in mind, that the relation 

 which the internal organization bears to the external structure is the only- 

 sound way of understanding the true relation of individual parts to the whole. 



In the genus Talitrus the appendages posterior to the powerful mandibles 

 appear to be strengthened by an internal process on either side, which is 

 produced until the two meet and form a ring. It is this ring that we contend 

 to be the homologue of the three posterior segments of the cephalic division : 

 that it is dorsal and not sternal, is demonstrated, we think, from the fact that 

 the nervous cord passes through the hollow, though to accomplish this a con- 

 siderable depression from its normal direction is produced. 



Thoracic segments (Pereion*). The seven annules which posteriorly 

 follow the cephalic portion are in the higher order protected by the carapace. 

 These become less so in the descending order ; and in the Amphipoda each 

 segment is formed into a perfect ring, analogous in appearance to the abdo- 

 minal segments in Macroura. 



The anterior of these thoracic segments differs in its position from those 

 which are posterior, by the circumstance, that the anterior margin overrides 

 the posterior edge of the cephalic, whereas in all the subsequent ones the 

 anterior dips beneath the posterior edge of the annule immediately pre- 

 ceding, the two margins being united by a thin membrane sufficiently elastic 

 to admit of one plate passing to a small extent beneath the next. 



The several appendages supported by these segments are locomotive in 

 their character, sometimes more perfectly perambulatory, at others adapted 

 for climbing and grasping, under which character the two anterior are most 

 constant in their adaptation ; and the probability is that they are never used 

 except as supplying organs to the mouth, unless to assist in climbing occa- 

 sionally. 



On each side of the several annules of the thorax, the Amphipoda are 

 remarkable for the development of a large scaliform appendage, which Prof. 

 Milne-Edwards, and hitherto every author after him, consider to be epimeral 

 or side-pieces of the dorsal arch, of each respective segment, remaining 

 unfused. These so-called epimerals we exclude from being a portion of the 

 true segment, believing them, as we think we shall be able clearly to demon- 

 strate in the proper place, to be the first joints or coxae of the legs. 



Abdominal ser^ments (or pleon-f). — The next succeeding seven rings form 

 the so-called abdomen of all later carcinologists, but they support three very 

 ■distinct kinds of appendages. 



In the Brachyura the appendages are all of one sort, and these all present 

 only in the female, and are adapted to a special function connected with the 

 process of reproduction. In the male they are absent, except the two an- 

 terior pairs, which are modified so as to adapt them to fulfil the office of 

 intromittent organs. As we descend in the scale from perfect development, 

 we perceive that the posterior annules are constructed and arranged so as 

 to become a tail piece, and a powerful and efficient organ it is in the Ma- 

 croura and Anomoura, which enables the animal to dart or swim through the 

 .water with considerable force and velocity. 



The number of segments which are arranged to complete the caudal 

 appendage differs in separate orders. In the Brachyura there is but one ; 

 •among the Macroura the two last segments are so arranged ; but among 



* Trom Trepaiou), to walk about : pereion, part which supports the walking legs. This and 

 th€ following are suggested instead of the old and incorrect synonyms of thorax, abdomen, &c 

 t From TrXew, navigo : pleon, part which supports the swimming legs. 



