The remaining groups are dealt with under the headings Entomostraca and Malacostraca, the annectent group Leptostraca being included in the former.
However, owing to the evolution of the vertebral column in various directions, and to the inconstant state of things in certain annectent groups, it is not possible, it seems, to apply the vertebral characters to taxonomy with that rigidity which E.
Credner, basing his views on the discovery by him of various annectent forms between the Stegocephalia and the Rhynchocephalian reptiles, has proposed a class, Eotetrapoda, to include these forms, ancestors of the batrachians proper on the one hand, of the reptiles proper on the other.
In most of the older classifications great importance was attached to these physiological characters, and a number of genera were established which, owing to the numerous annectent forms which have since been discovered, must be abandoned.
Cope (16) regarded the Apoda as the extremes of a line of degeneration from the Salamanders, with Amphiuma as one of the annectent forms. In the opinion of P. and F.