BLOOD SYSTEM. 



36 



culum or gill cover. Lift this and look in; we see se^ 

 eral rows of red gill fringes, between which there ai 

 openings into the throat — gill slits. Look next into th 

 throat ; we see several cartilaginous arches, gill arche 

 with gill slits between. On these arches are fixed th 

 gill fringes, extending backward and outward undc 



Fig. 250. — Head of a fish with parts cut away so as to show the brain, spin 

 cord, and the gills : cb, cerebellum ; op, optic lobes ; cr, cerebrum ; olj 

 olfactory lobes ; ol/org, olfactory organ. The opercle is removed ar 

 the gill fringes partly removed, so as to show the four gill arches. 



the opercle. The fringes are thin, flat plates fixed t 

 the gill arches, like the teeth of a fine comb to its sten 

 They are very thin, very numerous, and arranged in tw 

 rows (Fig. 251). The purpose is to make as much sui 

 face as possible. They are intensely red, because th 

 blood is profusely distributed in them. The blood passe 

 along each arch, and is thence distributed on the fringe: 

 Mechanics of Breathing. — The mouth is opene 

 and the throat enlarged and filled with water. By shui 

 ting the mouth and contracting the throat the water i 



