the sedentary degenerate fixed adult Ascidian with little or no trace of the Chordate characteristics so marked in the earlier larval stage (see E and A, Fig. 26). The free-swimming tailed larva shows the Ascidian at the highest level of its organisation, and is the stage that indicates the genetic relationship of the Tunicata with the Vertebrata. In some Ascidians with more food-yolk in the egg, or in which {62}the development takes place within the body of the parent, the life-history as given above is more or less modified and abbreviated, and in some few forms the tailed larval stage is missing. Some exceptional cases of development will be noted below under the groups to which they belong. The remarkable life-history of the typical Ascidian, of which the outlines are given above, is of importance from two points of view:— 1. It is an excellent example of degeneration. The free-swimming larva is a more highly developed animal than the adult Ascidian. The larva is, as we have seen, comparable with a larval fish or a young tadpole, and is thus a Chordate animal showing evident relationship to the Vertebrata; while the adult is in its structure non-Chordate, and is _on a level_ with some of the worms, or with the lower Mollusca, in its organisation, although of an entirely different type. 2. It shows us the true position of the Ascidians (Tunicata) in the animal series. If we knew only the adult forms we might regard them as being an aberrant group of Worms, or possibly as occupying a position between worms and the lower Mollusca, or we might place them as an independent group; but we should certainly have to class them as Invertebrate animals. But when we know the whole life-history, and consider it in the light of "recapitulation" and "evolutionary" views we recognise that the Ascidians are evidently related to the Vertebrata, and were at one time free-swimming Chordata occupying a position somewhere below the lowest Fishes. {63}CHAPTER III TUNICATA (_CONTINUED_) CLASSIFICATION: LARVACEA—APPENDICULARIANS—STRUCTURE, ETC.—ASCIDIACEA—SIMPLE ASCIDIANS—SPECIFIC CHARACTERS—COMPOUND ASCIDIANS—GEMMATION—MEROSOMATA— HOLOSOMATA—PYROSOMATIDAE—THALIACEA—DOLIOLIDAE—SALPIDAE—GENERAL CONCLUSIONS—PHYLOGENY. We now turn to the systematic classification of the group; and further details of structure or function, points of interest in the life-history such as budding and the formation of colonies, the habits and occurrence, and other peculiarities such as phosphorescence, will all be noted under the orders, sub-orders, families and genera in which they occur. CLASS TUNICATA.