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 Coelenterates, 
it may be carried on in the water-vascular tubes of those 
animals, while in the so-called ‘‘ 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 
traly 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 back and 
forth, slowly or suddenly, according to the degree of tran- 
quillity or disquietude of their possessors. 
