Page:Natural History, Mollusca.djvu/134

122 seized by the papillæ of the tongue, which, being sharp and directed backwards, continually drag by a kind of peristaltic movement the alimentary materials into the œsophagus."

The animals of this family have a peculiar elegance, delicacy, and beauty. They have the branchiæ arranged along the sides of the back. In one genus, which links this family with the preceding, these organs are branched, resembling little leafless trees; but in general they are warts of a long oval or spindle shape, pointed at the extremity. Their surface is covered with strong cilia, which constantly maintain a vigorous vibration, by the action of which currents of the surrounding water are continually poured along each of the papillæ, as these organs are named, affording the necessary oxygen for renewing the vital power of the blood. There is reason to believe, however, that the whole surface of the body, which is also covered with cilia, assists in respiration.

The papillae are permeated by a canal, which in many of the species is brilliantly coloured, contributing largely to the exquisite beauty of these little animals. This canal is connected with the stomach and the digestive function, supplying the place of the liver in other animals. In the principal genus of the family, each papilla is furnished with a curious organ; it is a little oval bag placed at the extremity of the papilla, and opening by a very minute aperture at its tip. Within the bag