414 THE AMERICAN MONTHLY [Dec. 



C. The Mouth, — externally is bounded by two jaws 

 tli(j Free bone in the upper is the pre /)i.axillary, the lower 

 tiie de7itary (a part of the mandible of higher vertebrates). 

 Note that the lower jaw is longer than the ui>per. Do 

 both jaw-bones bear teeth? What is the shape and 

 position of the teeth? Do other bones of the mouth bear 

 teeth? Examine the intt-rior of the mouth, note that its 

 roof is entirely below the level of the nose, eyes and 

 ■cranium. Study the sides and floor of the chamber, is 

 there a fleshy tongue? Locate the hyoid bone in the 

 •centre of the floor; and the pairs of bones running frc-ra 

 it obliquely backward, then arching dorsally to run for- 

 ward and attach to the roof of the mouth posteriorly, 

 these are the branchial or gill arches', count tliem. Note 

 the openings, gill-slits between them, leading to the out- 

 side water; cut away the side of the mouth soas to enalile 

 you to examine the gill arches better; note in doing so: 

 the operculum, a fiat thin bony flap on the side of the head 

 -•and behind, which covers the gills; it is open posteriorly 

 to let the water that passes over the gills escape. How 

 do you imagine that the operculum benefits the smelt? 

 Cut off one of the gill-arches and examine it in j)erfectly 

 -clear water; note on its front side the row of fine deli- 

 cate bones, gill-rakers, which stand projecting into the 

 douth cavity in such a position as to strain the water and 

 retain any particles of food, and on the hind side the masses 

 of d^^ep red ^/7/.s-. Separate the latter carefully and prove 

 that they are made up of gieat numbers of delicate Jila- 

 iments all of them alike. Remove one of the filaments and 

 *pe its central stt m and nnn;eious f-ninll .^ide branches 

 containing the capillaries in whicii the blood is aerated. 

 The gill rakers and thefilaments are carried onbonesthat 

 support the arch and the chief blood vessels lie close to 

 these l)cnes and follow their course. 



I'he throat or hinder ])avt of the mouth chamber of 

 the fish is devoted to the function of respiration; in the 



