HOW ANIMALS BREATHE. 453 



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 sui'face 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 



Fig. 6. Phtixopoda. Fairy Shrimp (Chirocephalus diaphanus). (After Baird.) 



annelids breathe partly by currents of water passing through the in- 

 testine. A slight modification of the lining of the food-tract is found 

 in the larva? 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 respira- 

 tory function. 



Many fishes swallow air as an aid to respiration. In the loach 

 ( Cobitis fossilis), and numerous others, it is certain that this air trav- 

 erses 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 rela- 

 tion 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 ren- 

 dered 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. 



