OBJECTIONS TO EVOLUTION 177 



called water-breathers, and those that breathe atmos- 

 pheric air are called air-breathers. 



Many of the simple aquatic forms absorb oxygen 

 through the general surface of the body; others have 

 tubes that carry aerated water, through the walls of 

 which tubes oxygen is absorbed. 



Some marine worms have tufts of gills distributed 

 externally along the sides of the body. Bivalve 

 mollusks have two flat gills on each side of the body. 

 Aquatic gasteropods sometimes have tufts of gills 

 similar to those of worms, or comb-like ciliated gills 

 in a cavity behind the head to which the water is ad- 

 mitted by a siphon. 



"The cuttle-fish has flat gills covered by the man- 

 tle ; but the water is drawn in by muscular contrac- 

 tions instead of by cilia." "The gills of lobsters and 

 crabs are placed in cavities covered by the sides of 

 the shell (carapace); and the water is brought in 

 from behind by the action of a scoop-shaped process 

 attached to one of the jaws, which constantly bales 

 the water out at the front." * 



Most fishes have several gills with comb-like fringes 

 lying close together under a covering at each side of 

 the head; others have pouch-like gills scattered along 

 the side of the neck. The Amphioxus absorbs air 

 through the mucous membrane of the-pharynx. 



The breathing apparatus of air-breathers is either 

 tubes, sacs or lungs. 



Insects have a system of air-tubes extending through 

 the body, which have several external openings along 

 the sides of the body. 



In spiders the tubes are connected with air-sacs 

 which aid in breathing. Scorpions breathe by means 

 of four pulmonary sacs. 



Snails have a cavity on the right side of the neck 

 12 * Orton's Zoology. 



