But structures essentially similar (called avicularia, or "bird's-head processes") are developed from the surface of the compound masses of certain of the highest of the polyp-like animals (viz.

the Polyzoa or, as they are sometimes called, the Bryozoa).
These compound animals have scattered over the surface of their bodies minute processes, each of which is like the head of a bird, with an upper and lower beak, the whole supported on a slender neck.

The beak opens and shuts at intervals, like the jaws of the pedicellariae of the echinus, and there is altogether, in general principle, a remarkable similarity between the structures.

Yet the echinus can have, at the best, none but the most distant genetic relationship with the Polyzoa.

We have here again therefore complex and similar organs of diverse and independent origin.
[Illustration: BIRD'S-HEAD PROCESSES VERY GREATLY ENLARGED.] In the highest class of animals (the Mammalia) we have almost always a placental mode of reproduction, _i.e._ the blood of the foetus is placed in nutritive relation with the blood of the mother by means of vascular prominences.