ZOOLOGY. 793 Savigiiy (ISIG), who showed that the mouth- parts of insects, crustaceans, etc., were modified limbs, and tliis yave a clue to the segmental nature of the head of these animals, which is composed of a number of segments, difl'crcnt in cacli class, as shown by the later researches of embryologists. Savigny also taught that as a rule a segment bears but a single pair of ap- pendages, while Audouin showed that the form of the arthropodan body was due to the atrophy of certain segments, or parts of segments, and the correlated hypertrophy of others. Other im- portant advances in classilication and anatomy were made by the labors of II. Milne-Edwards, the successor of Cuvier, who established the prin- ciple of the division of physiological labor, sepa- rated the Tunicata from the Jlollusca, divided vertebrates into those with and without an al- lantois_, and confirmed De Blainville's separation of amphibians from reptiles. During the middle of the nineteenth century the great German anatomist, Miillcr, made ex- tensive investigations on the eiiiliryology and metamorphoses of the echinodcrms. revolution- ized the classification of the fishes, and placed physiologv' on a new and higher basis. Jlean- wliile Vaughan Thompson (1830), by his dis- covery of the nauplius larva of the barnacles, had shown that instead of being worms they were genuine, but greatl.y degraded, crustaceans. In 1848 Leuckart broke up the Kadiata of Cuvier, and separated the Co;lenterata from the echine- derniR, regarding them as a distinct branch or type. Wiegman transferred the Rotifera from the Protozoa to Vermes; Siebold (1848) having established the group of Protozoa, showed their one-celled structure, and founded the type of Artliropoda ; while Rudolplii, Leuckart, and Sie- bold pointed out the worm-like nature of the flat- worms, now referred to a separate phylum. The animal nature of sponges was not proved until after the middle of the nineteenth century, through the researches of Lieberkiihn, Carter, Clark, and finally Haeckel, who. however, placed them with the ccelenterates, while their true po- sition as a distinct phylum was assigned them by Hy.itt. Through the investigations of Morse the Brachiopoda were removed from the Mollusea, as also were the Molluscoidca, and regarded as more nearly allied to the Annelida. Louis Agas- siz (q.v.). one of the foremost zoologists of his time (1807-73), greatly advanced our knowledge of the ca»lenteratcs. among his discoveries being that of the hydroid nature of Millepora. He worked on the embryology of the fishes and rep- tiles, his great work on fossil fishes bringing order out of a chaotic mass of unclassified material. He also indicated the chief faunal divisions of North America. From the middle of the nineteenth century there has been pub- lished an uninterrupted series of monographic anatomical and embryological works on single animals in the line of those of Reaumur, Lj'onnef, Btrauss-Durckheim. and Herold. among which should be indicated those of the numerous stu- dents of Johannes Miiller and Leuckart; the works of Agassiz and his 'students; of Huxley and his pupils; and of Lacaze-Duthiers and his successors. Among the most remarkable and fruitful ad- vances in embryology is Kowalevsky's discovery ZOOLOGY. (1860) of the presence of the notoehord and hol- low nervous system in the embryo and larva of ascidians, which resulted in the removal of the Tunicata from the MoUuscoida, and afterwards from the worms, to the neighborhood of the verte- brates; and to the establishment of the phylum of the Cliordata, as well as the breaking down of the distinction between vertebrates and inverte- brates; these, with the later researches of Sem- per, indicating the origin of vertebrates from some primitive annelid worm. The fact that the embrj'os of all animals above the Protozoa pass tlirough a gastrula stage led Haeckel to suppose that his hypothetical gastrea was the ancestral form of all the many-celled animals. The two founders of modern histotegy were Kcilliker and Leydig. The former, besides his early discovery of the mode of development of sperm cells, and the proof he alTorded of the ex- istence of animal cells destitute of a cell-wall, gave a great impulse to the general histology of vertebrates, and the latter to that of the inver- tebrates, especially the sense-organs. Their pu- pils and successors, by the invention of refined nibthods and the use of reagents, especiallv by the invention of the microtome, have pushed fruitful investigations in embryological and mor- pliological directions, greatly extending our knowledge of reproduction and cell-division, and laying the foundation for future special research in comparative physiology, especially of the brain, and in psychologj', bearing especially on reflex, instinctive, and mental actions. The studies of the earlier histologists and em- bryologists led to the foundation of cytology, or cell-science, and to a realization of the complex nature of protoplasm; work in which Fleming, Strasburger, E. Van Beneden, the two Hertwigs, Roux, Boveri, Wilson, and others took a leading part. On the results of their labors are based the theory elaborated by Weismann that the par- ticles of the chromatin of the nuclear substance (chromosomes) are the bearers of heredity. A new department of cytology is that of the me- chanics of development (q.v.), which is in the line of mechanical or dynamic evolution, taught by Lamarck, H. Spencer, Ryder, Cope, Hj-att, Roux, and others. The metamorphoses of animals, especially those of insects, were in the beginning described by Lyonnet, Rocsel, De Ciecr. Rf'aumur, and others, and in recent times by Fritz Jlfiller, who pointed out the fact that the caterpillar and maggot form of larva are secondary, the most gen- eralized and primitive insects not undergoing a metamorphosis. Miiller also worked out the metamorphosis of certain Crustacea, and our knowledge of the larval forms of these animals has been greatly extended by Dohrn. Brooks, Smith, and others, with the result that the nau- plius stage is now regarded as also an acquired one, and not a primitive feature. The most notable advance in our knowledge of the exact mode of metamorphosis in the most highly spe- cialized insects, especially the flies, was made by Weismann in 1804. His demonstration of the making over of the body by the histolysis of the larval tissues, and the formation of the tissues of the adult from imaginal buds, brought about the final overthrow of the preformation notion. Tlirough the labors of Barrande. Beecher. and others, the larval stage of trilobites and of Limu-
