Wilder.] 186 [April 5, 



homology is perfect, but as he discriminates between the radiates 

 and the bilateral animals of the other branches, it would seem better 

 to call this relation of opposite spheromeres, simply symmetry, or per- 

 haps pseudantitropy, and to confine antitropy to the three higher 

 branches; for otherwise, we should have to devise another and differ- 

 ent term for the relation in them ; laterality does not seem quite suit- 

 able, because, as used by Agassiz, (Rem. on 278, 279), "it relates to 

 the disposition of organs upon any two sides of the body, without 

 reference to symmetry " ; and it is not evident that the idea of real 

 homology is included in this laterality. 



Platetropy. 



Symmetrie, Fr. — Symmetrie, Ger. — Symmetria, Lat. — Symmetry, (in 

 part), Most authors. — Respective symmetry, Architecture. — Lateral 

 symmetry, Ok., 285, Par. 2114, (1810). — Bilateral symmetry, Ag., 

 (Rem. on) 298, (1861). — Homologie symetrique laterale, Foltz, 39, 

 51, (1863).— Bilaterality, Clark, 211, 265, (1865).— Latitypy, Wild., 

 45, 14, (1865).— Right and left symmetry, Wy., 55, 254, (1867).— 

 Latipolar homology, Wild., 58, Lect. 1, (1867). — Lateral homology, 

 Miv., 278, 119, (1870).— Lateritypy, Coues, 70, 151, (1870).— Trans- 

 verse symmetrical repetition, Coues, 70, 150, (1870). — Transverse 

 polar antagonism, Coues, 70, 150, (1870). — Latitropy, Wild., 74, 

 fere, (1871). . 



Definition. The antitropical relation between parts upon oppo- 

 site sides of the longo- vertical plane. 



Example. The relation between the right and left ear, nostril or 

 kidney. 



Remark. This kind of symmetry is so evident with the majority 

 of vertebrates and articulates, and with many mollusks and appar- 

 ently with some radiates, that it is generally recognized and even 

 thought to be absolute in some cases. But the perfect symmetry of 

 crystals is never realized, according to high authorities, and Wy- 

 man, 55, 247, says "it may be doubted whether absolute symme- 

 try exists anywhere." In 312, I have given instances of deviations 

 from symmetry from many groups of animals, and have thus tried to 

 bridge over from one side the gulf which is generally supposed to 

 wholly separate lateral symmetry (platetropy) from longal symmetry 

 (meketropy) ; the corresponding work from the other side will consist 

 in the presentation of evidence of the close homology which, in many 

 cases, exists between parts at the two ends of the body; and the first 



