On account of the presence and mode of origin (from the gut-wall) of this organ Bateson introduced the term hemichorda as a phyletic name for the class Enteropneusta.
Each of these schools impresses its pupils, in the case of the birds, with its own stamp, but there are many combinations, since in the course of phyletic development many a group of birds has exchanged one school for another.
In general we find an analogy between the development of groups and of organs; we discover that each phyletic branch of certain organisms traverses a geologic career comparable to the life of an individual, that we may often distinguish, especially among invertebrates, a phase of youth, a phase of maturity, a phase of senility or degeneration foreshadowing the extinction of a type.
Huxley, the sporosac is the starting-point of an evolution leading up through the various types of gonophores to the free medusa as the culminating point of a phyletic series.
The absolute agreement in the results independently obtained by these various investigators, the interpretation of individual development as the guide to phyletic development, the demonstration of continuous genetic series, each mutation falling into its proper place and all showing a definite direction, constitute contributions to biological philosophy of the first importance, which have been little known or appreciated by zoologists because of their publication in monographs of very special character.