114 



COMPARATIVE ZOOLOGY. 



water is drawn in by muscular contractions instead of by 

 cilia. The end of the siphon through which it is ejected 

 is called the funnel. The gills of Lobsters and Crabs are 

 placed in cavities covered by the sides of the shell (cara- 

 pace); 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. 



The perfection of apparatus for aquatic respiration is 

 seen in .Fishes. The gills are comb-like fringes supported 

 on four or five bony or cartilaginous arches, and contain 

 myriads of microscopic capillaries, the object being to ex- 

 pose the venous blood in a state of minute subdivision 

 to streams of water. The gills are always covered. In 

 bony fishes they are attached to the hinder side of bony 

 arches, all covered by a flap of the skin, supported by 

 bones (the gill-cover, or operculum), and the water escapes 

 from the opening left at its hinder edge. In Sharks, the 

 gills are placed in pouches which open separately (Figs. 

 164 and 287). The act of "breathing water" resembles 

 swallowing, only the water passes the gills instead of en- 

 tering the gullet. 



(2) Air-breathers have 

 trachece, or lungs. The 

 former consist of two 

 principal tubes, which 

 pass from one end of 

 the body to the other, 

 opening on the surface 

 by apertures, called spir- 

 acles, resembling a row 

 of button - holes along 

 the sides of the thorax 

 and abdomen, arid rami- 



FiG. 79._Spiracle of an Insect, x 75. f y . fig throngll the gmall- 



est and most delicate organs, so that the air may follow 



