ON THE BRITISH EDRIOPHTHALMA. 3ff 



mentary ; but as far as our experience goes, it is never developed into a per- 

 fectly simple foot. The nearest approach may be in Tetromatus. 



These two pairs of members are formed most commonly upon the same 

 type, those of the same pair are invariably alike. Once or twice we observed 

 indications of a variety of form between those of the same pair, but these we 

 were induced to consider as the result of an abnormal condition of the part 

 rather than a constant feature in the species. 



Even between the sexes the form of these members exhibits a very marked 

 similarity, though the rule is not constant. We see in Orchestia littorea that 

 the second pair of gnathopoda in the male are furnished with large powerful 

 claws ; whereas in the female they are scarcely more than rudimentary, and 

 assimilate in form to those found in the larva of this species. The realiza- 

 tion of the same may be found in a few other species, but still the prevailing 

 rule admits of little variation even where any exists. 



The Pereipoda*, or walking feet. — The two next succeeding pairs are the 

 first true perambulating feet, and are always developed simple in the Am- 

 phipoda, unless there may be an exception in the genus Phrosina, The 

 first homologizes with the great claw in the Macroura and Brachyura; and 

 both are in all the swimming Amphipoda less important in their peculiar 

 character than either those which are anterior or posterior to them ; but in 

 those which use them more in walking, which include many of the Corophiidce, 

 they are larger and stronger. Their action is directed forwards, similarly to 

 the two gnathopoda or anterior pairs of feet. 



The three next pairs of legs are the last belonging to this portion of the 

 animal, and are the powerful perambulators in Amphipoda ; generally the last 

 is the longest, but not invariably so ; in Phoxus it is almost obsolete. They 

 difier from the anterior in being directed backwards, and having each the 

 thigh or basal joint developed into a scale-like process. 



Among the more important features which are peculiar to the legs of the 

 Amphipoda^ and perhaps to the whole of the legion of Edriophthalma, and 

 identify them as distinct from the Podophthalma, is, that every joint is so 

 constructed that the whole leg can move only in its own plane. The legs of 

 the Podophthalma are arranged to admit of greater freedom in their action ; 

 they can bend them in almost any direction. Independently of this pecu- 

 liarity, there are others equally characteristic of the order. 



The separate parts of which the leg is constructed are unequal in their 

 respective lengths as well as different in form in the separate orders. The 

 basal joint in Podophthalma is extremely short and unimportant in appear- 

 ance, whereas among the Amphipoda it becomes perhaps the most powerful 

 and conspicuous of any, as may be seen by reference to the table repre- 

 senting the homologies of the leg in Crustacea (PI. XVI. figs. 2, 3, &c.). 

 Moreover it is often so developed, as, when folded up, to receive the extre- 

 mity of the same leg within a groove, and sometimes, as in Acanthonotus, the 

 propodos is completely buried and protected from accident. 



The knee or bending articulation, which admits of one portion of the leg 

 being folded upon the other in the Brachyura, takes place between the meros 

 and the carpus : in the Amphipoda it takes place between the ischium and 

 meros ; but the greatest individuality in the character of the legs of the 

 Amphipoda proper, as we;ll as the Isopoda proper, and whichj we think, has 

 led to error in the appreciation of the true position of these creatures in 

 the class Crustacea, is to be found in the development of the coxa or first joint 

 of the leg; the epimerals of authors generally, and Prof. Milne-Edwards in 

 particular. 



* This includes the five posterior thoracic feet of authors. 



d2 



