ON THE BRITISH EDRIOPHTHALMA. Sjf 



those of Crustacea generally, and Amphipoda in particular, which is better 

 and we think fully explained in the table of the homologies in Plate XVI. 



2nd. That the hranchia is normally an appendage of the leg and attacked 

 to the COM.— This is readily observable in tiie Amphipoda, but not so di- 

 stinct in the higher types, inasmuch as the organ is developed within the 

 walls of the carapace and possesses an internal character. But this internal 

 character is one of appearance only, dependent upon the monstrous growth 

 of the carapace, which covers the rings and the branchial appendages also. 

 Therefore, whenever the anterior cephalic segments cease to be developed 

 into a carapace or protecting buckler, the branchial organs must be external, 

 which in reality is their homological position even in the highest developed 

 forms. 



In the Brachyura and Macroura the branchial organs are lodged in a 

 cavity formed by the carapace, but they are separated from the great cavity 

 containing the internal viscera by the wall of the segments belonging to the 

 (so-called) thorax. These segments are not complete in their structure, but 

 still they are a portion of the external skeleton, and the branchial organs 

 developed upon their outer surface are homologically the same as the bran- 

 chial sacs on the inner side of the coxa in the Amphipoda ; and the proba- 

 bility is that the disarrangement exists in the higher type, in order to meet 

 certain conditions which enable them to fulfil the more complete function 

 of internal gills. The typical character of the branchial organs in Crustacea 

 is an external apparatus. 



The coxa in the Brachyura is anchylosed with the segment of the body. 

 In Macroura it is free; consequently we can the more readily perceive the 

 attachment between it and the branchia. The flabella in the same orders, 

 which are nothing more than an altered gill, originates from the same joint, 

 and every fact proves to demonstration that the true homological position of 

 the branchia is in connexion with the coxa (PI. VIII. figs. 2, 3, 10). 



Admitting then that the branchial organs are appendages of the legs 

 attached to the coxae, we perceive at once, since they are attached to the (so- 

 called) epimerals, that these cpimerals must homologically be consonant with 

 the coxae o( the Macroura type, and therefore the first joint of each leg. 



3rd. The moveable power to the greatest degree is hetioeen the coxa and the 

 next succeeding joint, and never between the coxa, and the animal. — This is 

 most apparent in the Brachyura, where the coxa is fused with the segments 

 of the body. In the Amphipoda it is not fused, but fixed, and the greatest 

 freedom of motion to the legs is where the next joint is articulated with this, 

 which is so frequently close to the base, that it is highly probable that a 

 hasty examination of some of the more common species only, such as Talitrus 

 and Gammarus locusta, might delay the acceptation of a fact urged by an 

 unknown individual in opposition to the long-received idea propounded by 

 the highest authorities and admitted by all others (vide PI. XV. fig. 8). 

 But if the very transparent and by no means rare species of Gammarus 

 grossimanus be examined, the coxa will be found to have the scale-like form 

 developed to a moderate degree only ; and unlike most of the common 

 species, the basal joint articulates with the coxa almost at the extremity, 

 and gives to the latter so much the character of being a portion of the leg, 

 that if all others of the class had been the same, we doubt if any observer 

 would have thought of describing them as epimerals or side-pieces of the 

 true segments. This remark will also hold in relation to the three posterior 

 legs of Amphipoda generally, where the coxae are developed to a small 

 degree ; also in the group Aberrantia {Lcemodipodd), where each is fused with 

 the rest of the animal, as we find it is the casein Brachyura, a circumstance 



