40 AFFINITY OF THE TRILOBITES 



thus increased in an extraordinary degree, and the difference between thorax and abdomen 

 disappears entirely on a superficial observation. It is only the internal anatomical examina- 

 tion which determines the boundary of the two divisions, and exhibits the sexual opening 

 behind the eleventh ring, but this is seen even in Apns, exactly at the same spot at which 

 it is situated in Branclilpus. The two last rings are, however, excepted from this remark- 

 able and unique instance of the approximation of the abdomen to the type of the thorax, 

 these rings retaining the shape of the abdomen, but bearing no further organs of motion, 

 and terminating with simple horny appendages, as in Limnadia, or with articulated ones 

 as in Apus, and these are attached to the remarkably developed last joint of the body. 

 Between them is the anus. Brancldpus shows no trace of any of these characters, its 

 abdomen, consisting of nine joints, has no feet; and instead of the horny appendages, we find 

 in them either two large soft caudal fins, or, as in Artcmia, nothing whatever to represent 

 them ; but the females here also possess distinct egg-capsules at the commencement of the 

 abdomen, and the males smaller seminiferous sacs. Nothing of this kind, however, is found 

 either in Apii>i or in LiuiuacUa ; the males in the former bearing such a resemblance to 

 the females, that the former a few years ago were not known at all,* and Mr. Kollar, of 

 Vienna, was the first who discovered them ; f while in the latter the males possess organs 

 of copulation in the first modified part of the thorax, (or at least this is the case in a species 

 which has in consequence been detached to form the new genus Estheria). The females of 

 Apns, however, can easily be recognized by the sacs, which are situated at the eleventh 

 pair of feet, and which serve as the repository of the eggs, but are placed towards the back, 

 beneath the shield. 



The feet of these animals exhibit also a difference corresponding with that presented 

 by the structure of the body, both in the case of those which have shells and which are 

 without such defence. Tliey consist in all cases of soft, membranous lobes, which are 

 mei'ely supported by muscular bundles, the circumference of which is intersected at intervals 

 irregularly, and at the margin they are covered with long, fine, hairy fin-bristles. At the 

 inner side six principal lobes are seen, of which the first four are of nearly equal size in 

 Limnadia (Fig. 15, B) ; but in Ajms (Figs. 9, 10, 11) the first (B) differs very much, and the 

 succeeding ones resemble one another, only they become larger from the basis to the point. 

 In Branchipus, however (Fig. 12), they become smaller in the other direction ; and the fifth 

 lobe, the last but one, which is very long and small in Limnadia, is very broad and rounded 

 in BraticJripiis, and in Ajms is similar to the preceding lobes. The last, the sixth, lobe is 

 connected by a special joint with the rest of the foot, and is therefore more freely moveable ; 

 it has a long stretched, rudder-like form, and seems to be the most important of all the 

 divisions of the foot. Every foot, at the opposite outer side, bears a bladder-formed gill 

 (K, in the plate), and is also provided with broad lobes of membranes. Of these we only 

 find one very large lobe beneath the gill in Apus and Limnadia (L) ; but in BramUjms there 

 are two lobes (which, however, are both situated at the gill), one of them a large one, 



* In a work, otherwise very excellent, h\ E. G. Zaddack, (de Apod, cancriformis anatome et 

 evolutione, Bonn, 1841, 4to), these animals are described as hermaphrodites, which probably is only to 

 be attributed to a defective microscopical aualj-sis of the organs of generation. 



t 'Isis,' 1831, p. G80; Froriep's 'Notizen,' 1833, vol. xxxviii, p. 148, etc. Mr. Kollar had tlie 

 kindness to present me with a male specimen. 



