The snowy heron is a rare plume bird seen occasionally along the coast.
Mantle-skirt, allowing the g, Gill-plume (ctenidium).
The young lack the ornamental plume, and in them the head and neck are clothed with short black feathers, while the bill is yellow.
It is important, because such a concrescence is by no means universal, and does not occur, for example, in Mytilus or in Arca; further, because when its occurrence is once appreciated, the reduction of the gill-plates of Anodonta to the plume-type of the simplest ctenidium presents no difficulty; and, lastly, it has importance in reference to its physiological significance.
Pulmonata are widely distinguished from a small number of Streptoneura at one time associated with them on account of their mantle-chamber being converted, as in Pulmonata, into a lung, and the ctenidium or branchial plume aborted.