der—breathes entirely by its skin under ordinary circumstances. To expose more surface, the skin is greatly expanded, lying in ugly folds; while, to renew the contiguous water, the body is frequently arched and rolled from side to side, which motion, together with the aspect of the creature, has given it the above expressive name.
As the outer surface of the animal body may without any special modification have a respiratory function, so we find the same to be true of the food-canal. The body-cavity of the polyp and jelly-fish, with its tubes and branches, has in previous articles been described as the organ of digestion, circulation, and respiration in common. Many
annelids breathe partly by currents of water passing through the intestine. A slight modification of the lining of the food-tract is found in the larvæ of the dragon-flies, which, living in water, have respiratory villi in the rectum; and the sea-cucumber has foliated processes in its stomach, bathed by water-currents, which doubtless have a respiratory function.
Many fishes swallow air as an aid to respiration. In the loach (Cobitis fossilis), and numerous others, it is certain that this air traverses the stomach and intestines.
Special Organs of the Skin.—Of the special breathing-organs we will first consider those which are developed directly from the skin. They are especially characteristic of the invertebrates, as they include all the special respiratory apparatus of that division of the animal kingdom, whether adapted for water or air, and they are not found beyond this group.
Aquatic Organs of the Skin.—The higher radiates and the lower articulates possess an arrangement of tubes and vessels known as the "water-vascular" or "aquiferous" system. Although the relation of these vessels to the circulation is not fully determined, it is probable that they are chiefly or entirely for the purpose of conveying water inward, and thus constitute respiratory organs. They are rendered necessary because of the undeveloped condition of the blood-circulating system. Like other cavities, they are only an inflection of the surface-tissue.
This aquiferous system is of very great complexity and variety, and full description is impracticable. It is at once greatly developed in the sea-urchin and star-fish; and is especially complex in the sea--
- Other articles in comparative physiology are contained in the "Monthly" of April, June, and September, 1880; and August and September, 1881.