History of larval forms, pp. 459-511. _Branchiopoda_, pp. 459-465. _Malacostraca_, pp. 465-487. _Copepoda_, pp. 487-492. _Cirripedia_, pp. 492-500. _Ostracoda_, pp. 500-502. _Phylogeny of the Crustacea_, pp. 502-511. The formation of the germinal layers, pp. 511-521. Comparative development of organs, pp. 521-529. CHAPTER XIX. Poecilopoda, pp. 534-538. Pycnogonida, pp. 538, 539. Pentastomida, pp. 539-541. Tardigrada, p. 541. Summary of Arthropodan Development, pp. 541-543. CHAPTER XX. ECHINODERMATA. Development of the germinal layers, pp. 544-553. Development of the larval appendages and metamorphosis, pp. 553-573. Summary and general considerations, pp. 573-576. CHAPTER XXI. ENTEROPNEUSTA. Pp. 579-583. INDEX. Pp. 584-590. APPENDIX. EMBRYOLOGY. INTRODUCTION. Embryology forms a large and important department of Biology. Strictly interpreted according to the meaning of the word, it ought to deal with the growth and structure of organisms during their development within the egg membranes, before they are capable of leading an independent existence. Modern investigations have however shewn that such a limitation of the science would have a purely artificial character, and the term Embryology is now employed to cover the anatomy and physiology of the organism during the whole period included between its first coming into being and its attainment of the adult state. The subject-matter of the science of Embryology admits of a twofold classification. It may be placed under a series of heads, each dealing either with a special group of organisms, or with a special department of the whole science. If classified in the first of these ways the science will naturally be divided into an Embryology of Plants, and an Embryology of Animals; each of which admits of further subdivision. In the second way the subject falls under two primary heads; viz. Physiological Embryology and Anatomical Embryology. The present treatise deals only with the Embryology of Animals, and is further confined to those animals known as Metazoa. The science is moreover treated from the morphological or anatomical, rather than from the physiological side. The marvellous phenomenon of the evolution of a highly complicated living being from a simple undifferentiated germ in which it needs the aid of the most modern microscopical appliances to detect any visible signs of life, has not unnaturally attracted the attention of biologists from the very earliest periods. Before the establishment of the cell theory the origin of the organism from the germ was not known to be an occurrence of the same nature as the growth of the fully formed individual, and Embryological investigations were mixed up with irrelevant speculations on the origin of life[1].