In 1872 Fol[77] added largely to the knowledge of the Appendiculariidae, and Giard[78] to that of the Compound Ascidians. The latter author described a number of new forms and remodelled the classification of the group. The most important additions which have been made to the Compound Ascidians since Giard's work have been the species described by von Drasche,[79] from the Adriatic, and those discovered by the "Challenger" expedition.[80] The structure and the systematic arrangement of the Simple Ascidians have been discussed of recent years mainly by Alder[81] and Hancock, Heller,[82] Lacaze-Duthiers,[83] Traustedt,[84] Roule, Hartmeyer, Sluiter[85] and Herdman.[86] In 1874 Ussoff investigated the minute structure of the nervous system and of the underlying gland, which was first discovered by Hancock, and showed that the gland has a duct which communicates with the front of the branchial sac or pharynx by an aperture in the dorsal (or "olfactory") tubercle. In an important paper published in 1880, Julin[87] drew attention to the similarity in structure and relations between this gland and the "hypophysis cerebri" of the Vertebrate brain, and insisted upon their homology. Metcalf has recently added further to our knowledge on this and related matters. The Thaliacea or pelagic Tunicata have of late years been the subject of several very important memoirs. The researches {39}of Todaro, Brooks,[88] Salensky,[89] Seeliger,[90] Korotneff,[91] and others have elucidated the embryology, the gemmation and the life-history of the Salpidae; and Grobben, Barrois,[92] and more especially Uljanin,[93] have elaborately worked out the structure and the details of the complicated life-history of the Doliolidae. Finally we owe to the labours of Metschnikoff, Kowalevsky, Giard, Hjort, Seeliger, Ritter, Van Beneden and Julin, much detailed information as to development and life-history, the process of gemmation and the formation of colonies, which has added greatly to our knowledge of the position and affinities of the Tunicata and of their natural classification. STRUCTURE OF A TYPICAL ASCIDIAN.