ON THE BRITISH EDRIOPHTHALMA. 51 



the branchial organs, where the blood, which is much divided and exposed 

 to aeration, goes, we believe, direct to the heart, and then, without returning 

 again to these organs, passes on its way, carrying oxygen to the general 

 system. 



The Branchiae. — These are by no means the simple sacs that authors have 

 universally described them. They are situated upon the inner surface of 

 the coxae of the leg, and assume the form of leaf-like plates on each side of 

 the sternum, and are attached to every leg except the first in females, and 

 generally the last in males, though in Gavmmrus we have seen them present 

 in the male as well as the female, on the seventh, as shown in PI. XXI. fig. 3. 



The arterial course passes down on the side nearest the heart, and divides 

 itself as it proceeds along the internal labyrinth of the organ into many 

 streams, and passes out of the vesicle by an efl^erent course on the side 

 opposite to that on which it entered. 



The corpuscles never increase beyond one deep. Thus each of these 

 supposed oxygen carriers is brought into immediate contact with the thin 

 walls, which alone separate them from external atmospheric influences. The 

 branchiae homologize with the same organs in the higher orders of Crustacea, 

 and each may be viewed in the light of a solitary plate of one of those more 

 compound organs. In fact they bear an extremely close resemblance to the 

 branchiae of the Brachyura in the larval condition, before they assume 

 the foliaceous appearance of the perfect organ (PI. XVIII. fig. 10). 



The great difference in the general character appears to be derived mostly 

 from the appearance which the organs in the higher types assume of a resem- 

 blance to an internal position ; but this is a condition of appearance only, as 

 shown in an earlier portion of this paper; the branchiae are overcapped by 

 the monstrous production of the anterior cephalic segments, a peculiarity 

 which is not carried out in the Amphipodous order ; consequently the 

 branchiae are external and pendent in the water, and it is for their greater 

 protection that the coxae are developed into large scaliform plates. 



The internal structure of the branchial organs appears to be produced by 

 a thickening of a fibrous tissue in contact with the internal surface of the 

 walls of the organ (PI. XVIII. fig. 7). This appears to be carried out in 

 patches of an irregular form, but which correspond in their arrangement 

 with one another. These patches are thickest at their centre and thin out 

 towards their edges : the result is that a channel is left between each. All 

 the channels so formed are connected together throughout the whole organ, 

 and exhibit a continuous labyrinth in which the blood circulates in many 

 small streams (fig. 8). 



Should the animal become feeble, a gradual accumulation of corpuscles 

 may be discerned in different parts of the gills, mostly out of reach of the 

 stronger currents, which latter, as the vitality of the aniniul diminishes, can 

 be observed to lessen in force until it is propelled only by jerks, coexistent 

 with every pulsation of the heart ; and at length a throbbing without any pro- 

 gression of the corpuscles appears as the last effort of decaying circulation. 



The external form of the, organ varies but little: in Talitrus (PI. XVIII. 

 fig. 3) there appears a second of smaller dimensions, originating from a com- 

 mon base, the stalks being separated. Somewhat similar are they in the 

 branchiae of Sidcator arenarius (fig. 1), and would appear as if it were an 

 effort of nature to make a step towards the more foliaceous organs of the 

 higher types. In the Aberrantia we find that Caprella Pcnnantii (for in 

 this group, except in the genus Proto, there are but two sets attached to the 

 third and fourth segment of the pereion (thorax)) has the anterior branchia 

 round and much larger than the posterior, which is more cylindrical in form. 



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