Wilder.] 



180 [April 5, 



Single or Absolute or Tropical Homology. 



Although the detailed comparison of the membra with each other 

 was first made by Vicq d'Azyr, yet the germ of tropical homology 

 existed in all recognitions of the correspondence of the right and left 

 sides of the body; many and vague terms were employed (parallel- 

 ism, analogy, homology, correspondence, repetition) which did not 

 imply a difference between single and plural homology, or between 

 the different kinds of the former. I hope hereafter to show that the 

 same methods of comparison and argument are as applicable to single 

 as to plural homology ; and that cephalo-caudal repetition is compar- 

 able to dextro-sinistral repetition. 



Spherical Homology. 



Radiality, Ag., (Kem. on) 298, 279.— Radiation, Ag., 201, 292.— 

 Radial arrangement, Rol., 294, cxliii, clvi. — Radial symmetry, Hux., 

 251, 46. — Radiate symmetry, Ag., 202, 33. — Radial homology, Miv., 

 278, 119. — Spherical homology, Wild., 58, Lect. 1. 



Definition. The tropical relation between the morphically iden- 

 tical, converging spheromeres of a radiate animal. 



Remark. The above definition is chiefly based upon the presenta- 

 tion of the subject by Agassiz especially in 200, 3, pp. 79, 260, 261, 

 etc.; but there remains much to be done toward clearing up the confu- 

 sion in which the whole subject now rests. In the first place two 

 distinct ideas are included in the above list of terms ; radiality is a 

 general name for an abstract idea involving the plan of structure of a 

 branch of the animal kingdom; Agassiz admits, 200, 3, 209, 210 ? 

 211, that upon this essential plan of radiality may be superinduced 

 an apparent bilateral symmetry, but that he does not regard this as 

 constituting a true bilaterality is shown by his contrasting the Radi- 

 ates with bilateral animals, 200, 3, 260. 1 



But the very existence of such a radiate idea, is questioned by 

 Morse, 281, 163, Clark, 211, 128, Huxley, 251, 47, and Rolleston, 

 294, cxliii, who hold that the bilateral symmetry which is quite 

 prominent in the larvae of echinoderms is equally, if not more character- 

 istic of the branch ; some join the echinoderms with the worms, Rolles- 

 ton, 294, 152, note; indeed so widely do they differ from Agassiz 

 in respect to the classification of the invertebrates, that anything like 



1 Also by his remarks in the Report of the Trustees of the Mus. of Comp. Zool. 

 1868, p. 9. 



