Dealing with these creatures in the broad anatomical spirit with which he had studied the Medusæ, Huxley shewed the typical structure manifested in the different forms, and that was common to them and the Ascidians or sea-squirts of the seashore. In a second paper on "Appendicularia and Doliolum" he made further contributions to our knowledge of these interesting creatures. Appendicularia is a curious little Ascidian, differing from all the others in its possession of a tail. Earlier observers had obtained it on various parts of the ocean surface, but had failed entirely to detect its relationship to the ordinary Ascidians. Chamisso got it near Behring's Straits and thought that it was more nearly allied to "Venus's Girdle," a Cœlenterate. Mertens, another distinguished zoölogist, had declared that "the relation of this animal with the Pteropods (a peculiar group of molluscs) is unmistakable"; while Müller, a prince among German anatomists, confessed that "he did not know in what division of the animal kingdom to place this creature." Huxley shewed that it possessed all the characteristic features of the Ascidians, the same arrangement of organs, the same kind of nervous system, a respiratory chamber formed from the fore part of the alimentary canal, and a peculiar organ running along the pharynx which Huxley called the endostyle and which is one of the most striking peculiarities of the whole group. The real nature of the tail was Huxley's most striking discovery. He pointed out that ordinary Ascidians begin life as tiny tadpole-like creatures which swim freely by the aid of a long caudal appendage; and that while these better-known Ascidians lose their tails when they settle down into adult life, the Appendiculariæ are Ascidians which retain this larval structure throughout life. Von Baer had shown that in the great natural groups of higher animals some forms occur which typify, in their adult condition, the larval state of the higher forms of the group. Thus, among the amphibia, frogs have tails in the larval or tadpole condition; but newts throughout life remain in the larval or tailed condition. Appendicularia he considered to be the lowest form of the Ascidians, and to typify in its adult condition the larval stages of the higher Ascidians.