248 



[June 5, 



riiilit and left orojuii. The fundamental idea of the organs of organic 

 life involves the condition of symmetry. 



The other kind of symmetry, viz., that which is believed to exist 

 V)etween the fore and hind parts of the body is much less obvious, and 

 would in fact be generally overI(X)ked, the deviations from true sym- 

 metry being so great. In many Articulates, however, this deviation is 

 comparatively slight, especially in the genera Jiiera (Fig. 1), Oniscus, 

 Porci'Ilio, Asellus, C;/mot/wa, and other 

 Isopods, also among ]Myriapods, as in the 

 genera Scutir/era, Scolopendra, etc., in 

 which the limbs are repeated oppositely,^ 

 though with diflferent degrees of inetpal- 

 ity. from the centre of the body back- 

 wards and forwards. If to the general 

 symmetry of such fore and hind parts 

 we add certain details of structure occa- 

 sionally seen, especially the fact that 

 some of the worms and Crustaceans 

 have organs of special sense developed 

 in the last as Avell as in the first seg- 

 ments, the evidence that the fore parts 

 are r'epeated in the hinder becomes much 

 stronger.* 



Among Vertebrates, as will be seen 

 further on, the resemblance between the 

 fore and hind limbs is quite obvious, and 

 *''g- 1- the synnnetry of plan easily recognized ; 



but in the majority (jf animals, whether vertebrate or invertebrate, 

 fore and hind parts, though symmetrical in plan, actually differ largely 

 from each other, both in size and form, and thus present a distorted 

 synnnetry like that which has ah-eady been noticed between right and 

 lefl parts. The analogy holds still further, since the fore and hind 

 limbs, however widely they may differ in the adult, are as nearly alike 

 in the early embiyo as are corresponding limbs on the right and left 

 sides. In right and left parts, however, distorted synnnetry is the ex- 

 ception, while in the fore and hind jiarts of adults it is the rule. 



A sulHcient ex])]anation of this deviation from complete synmietry 

 is found in the circumstance that in right and left parts the functions 

 are generally similar and equal, while this is seldom the case in fore 

 and hind ones. Identical and homologous parts having similar and 



* Leuckart and Van Beneden liave shown that Mysis has an ear in the last seg- 

 ment, and 8chmidt has described an eye in the same jiart in Amphirora, a worm. 

 Nat. Hist, llev., Ajiril, 1862, p. 133. See also Quatrefages, Memoire sur la Faraille 

 des Polyophthalmiens, Ann. des. Sc. Nat. 3™' serie, T. xm, p. 5. 



