Next with regard to the mode of fixation. Attachment always takes place by the surface on which the mouth or its rudiment is situated, and the permanent alimentary canal opens on the opposite surface. In Pedicellina, the one case in which the larval digestive organs are known to become those of the first adult individual, this presupposes a rotation of the alimentary canal, in order to bring it into its new position.

It is well known that the larvae of other fixed animals may undergo a somewhat similar change. Thus those of Ascidians and of Barnacles fix themselves by their anterior end, and ultimately reach their adult form by performing a kind of a somersault. The process may perhaps be explained by supposing that some part of the anterior end or of the oral surface is specially sensitive, and that the larva fixes itself by that portion of its body which is best fitted for ascertaining which is the proper substance on which to fix.

Budding.—The formation of a new individual may take place by the outgrowth of part of the body-wall, as in Pedicellina (Fig. 243, p. [487]) and in Bowerbankia (Fig. 238, p. [480]). In Pedicellina a young stalk is formed by an outgrowth near one of the growing points, and the upper part of this outgrowth becomes constricted off to form the calyx. In other cases (cf. the growing ends of the branches in Fig. 237) a partition grows across the body-cavity at the growing edge of the colony, and so cuts off a part destined to become a new zooecium.

The zooecium formed in one of these ways acquires an alimentary canal by the formation of a polypide-bud, some stages in the growth of which are shown in Fig. 235 (p. [472]). Contrary to what happens in Coelenterates and Tunicates, in which the endoderm takes part in the budding, there is good reason for believing that in Polyzoa the polypide-bud is developed entirely from ectoderm and mesoderm.[[583]] The bud is a two-layered vesicle, attached to the inner side of the body-wall. Its inner layer is derived from the ectoderm, which at first projects into the body-cavity in the form of a solid knob surrounded by mesoderm-cells. A cavity appears in the inner, ectodermic mass, and the upper part of the vesicle so developed becomes excessively thin, forming the tentacle-sheath, which is always developed in the condition of retraction. The lower part becomes thicker; its inner layer gives rise to the lining of the alimentary canal, to the nervous system, and to the outer epithelium of the tentacles, which grow out into the tentacle-sheath (cf. Fig. 235). The outer layer gives rise to the mesodermic structures, such as the muscles, connective tissue, and generative organs.

These processes are fundamentally similar, whether in the metamorphosed larva, in a young zooecium, in an old zooecium after the formation of a "brown body," or in the germinating statoblast of the Phylactolaemata.

CHAPTER XIX

POLYZOA (continued)

CLASSIFICATION—GEOGRAPHICAL DISTRIBUTION—PALAEONTOLOGY—METHODS FOR THE EXAMINATION OF SPECIFIC CHARACTERS—TERMINOLOGY—KEY FOR THE DETERMINATION OF THE GENERA OF BRITISH MARINE POLYZOA

Our account of the Polyzoa would be manifestly incomplete without some reference to the systematic arrangement of these animals. An outline of the principal groups has been given on p. [475]. So far, the classification is easy, but it is otherwise when we attempt to subdivide most of the groups any further.

Systems of classification which depend exclusively upon the external characters of animals have been repeatedly shown to be unsatisfactory. Now with regard to the Polyzoa, not only is it the case that the great majority of forms are only known in their external characteristics, but current systems of classification cannot be regarded as final, because it is not yet certain which of the external features have most systematic value. Two obvious points can be at once selected—namely, the character of the zooecium and the character of the entire colony. One or two instances will serve to show what different results are obtained by depending exclusively on either of these characters by itself.