COMPARATIVE ANATOMY OF ORGANS. 637 



Organs of Respiration. Always in intimate relation with 

 the circulatory system are the means of respiration. The 

 process may be carried on all over the body in the simple 

 animals, such as Protozoa or sponges, or, as in Ccelenterates, 

 it may be carried on in the water-vascular tubes of those 

 animals, while in the so-called te respiratory tree" of Echin- 

 oderms it may go on in company with the performance of 

 other functions by the same vessels. Respiration, however, 

 is inclined to be more active in such finely subdivided parts 

 of the body as the tentacles of polyps, of worms, or any 

 filamentous subdivisions of any of the invertebrates ; these 

 parts, usually called gills, though only the gills of fishes are 

 truly such, present in the aggregate a broad respiratory sur- 

 face. Into the hollows of these filamentous processes, 

 which are usually extensions of the body-walls, blood is 

 driven through vessels, and the oxygen in the water bathing 

 the gills filters through the integument, and immediately 

 gains access to and mixes with the blood. 



The gills of the lower animals appear at first sight as if 

 distributed over the body in a wanton manner, appearing 

 in some species on the head, in others along the sides of the 

 body, or in others on the tail alone ; but in fact they always 

 arise in such situations as are best adapted to the mode of 

 life of the creature. 



The gills of many of the lower animals afford an admira- 

 ble instance of the economy of nature. The tentacles of 

 polyps, polyzoans, brachiopods, and many true worms serve 

 also, as delicate tactile organs, for grasping and conveying 

 food to the mouth, and often for locomotion. The suckers 

 or " feet" of star-fish or sea-urchins also without doubt 

 perform the office of gills, for the luxuriously branched, 

 beautifully-colored tentacles of the sea-cucumber are simply 

 modifications of the ambulacral feet. One of the readiest 

 ways of judging of the mental condition, so to speak, of a 

 worm, such as Sabella or Terebrella or of a polyzoon or a 

 brachiopod, is to watch the movements of their beautiful 

 delicate gills, which are thrust in or out, waved hack and 

 forth, slowly or suddenly, according to the degree of tran- 

 quillity or disquietude of their possessors. 



