[33] Philosophical Transactions, 1865 and 1866. [34] Loc. cit. Zweit. Abh. Pl. i., figs. 8 and 9. [35] Thomson, on the Embryology of the Echinodermata, _Natural History Review_, 1863, p. 415. See also Agassiz, “Embryology of the Starfish,” p. 62. [36] A. Agassiz, Embryology of Echinoderms, p. 18. [37] Hincks. British Hydroid Zoophytes, pp. 120-147. [38] Zeits. für Wiss. Zool. 1864, p. 228. [39] Introduction to Entomology, 6th ed. vol. i. p. 61. [40] Métamorphoses de l’Homme et des Animaux, p. 133. See also Carpenter, Principles of Physiology. 1851, p. 389. [41] Darwin, Origin of Species, 4th ed. p. 532. [42] Principles of Biology, vi. p. 349. [43] For differences in larva consequent on variation in the external condition, see _ante_, p. 61. [44] See Hincks. British Hydroid Zoophytes, P. lxii. Agassiz, Sea-side Studies, p. 43. [45] See Newport, Phil. Trans., 1832. [46] Linnean Transactions, 1862. [47] Origin of Species, 4th ed., pp. 14 and 97. [48] On the Alternation of Generations. By J. J. Steenstrup. Trans. by C. Busk, Esq. Ray Society. 1842. [49] Zeit. für Wiss. Zool. 1863. [50] Mém. de l’Acad. Imp. de St. Pétersbourg. vol. xv. 1870. [51] Of course all animals in which the sexes are distinct are in one sense dimorphic. [52] “There is no such thing as a true case of ‘alternation of generations in the animal kingdom;’ there is only an alternation of true generation with the totally distinct process of gemmation or fission.”—HUXLEY _on Animal Individuality_, Ann. and Mag. of Nat. Hist. June 1852. [53] Prince Hohenstiel Schwangau, p. 68. [54] Journal of the Royal Institution. April 1873. [55] “Embryology of Echinoderms,” l. c. p. 15. [56] Mr. and Mrs. Agassiz: “Sea-side Studies,” p. 139. [57] l. c. p. 138. [58] Wien. Zool. Bot. Gesells, 1869. [59] Linnean Transactions, 1863. [60] Linnean Transactions, 1866, vol. xxv. [61] Linnean Transactions, vol. xxiv. p. 65. [62] Siebold und Kolliker’s Zeitschr. f. Wiss. Zool., 1864. [63] Linnean Journal, vol. xi. [64] Facts for Darwin, p. 120. [65] A still nearer approach is afforded by the genus _Peripatus_, which since the above was written has been carefully described, especially by Moseley and Hutton. There are several species, scattered over the southern hemisphere. In general appearance they look like a link between a caterpillar and a centipede. They have a pair of antennæ, two pairs of jaws, and (according to the species) from fourteen to thirty-three pairs of legs. They breathe by means of tracheæ, which open diffusely all over the body. [66] Unters. üb. die Entwick, und den Bau der Gliederthiere, p. 73. [67] Linnean Transactions, v. xxii. [68] Facts for Darwin, trans. by Dallas, p. 118. See also Darwin, “Origin of Species,” p. 530. 4th ed. [69] Mem. Peabody Academy of Science, v. I. No, 3. [70] Wien. Zool. Bott. Gesells. 1869, p. 310.