414 THE AMEHICAN MONTHLY [Dec. 



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

 the lice bone in tlie upper is the pre /n.axillary, the lower 

 the deiitary (a part of the mandible of higher vertebrates). 

 Note that the lower jaw is longer than the U[)per. Do 

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

 position of the teeth? Do other bones of the month hear 

 teeth? Exaniine the interior of the month, 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 hyold hone in the 

 centre of the floor; and the pairs of bones running fr(>m 

 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 them. Note 

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

 side water; cut away the side of the mouth so as to enable 

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

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

 :«ind l)ehind, 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 perfectly 

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

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

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

 retain any particles of food, and on the hindside themasses 

 of (h^ep red gills. Separate the latter carefully and prove 

 thai they are made up of great numbers of delicate Jila- 

 ^lefits all of them alike. Remove one of the filaments and 

 isee its centra] stem and nunieious t-mall .^ide branches 

 containing the capillaries in whicii the blood is aerated. 

 The gill rakers and thetilaments are carried onbones ihat 

 su])j)ort the arch and the chief bloodvessels lie close to 

 these bcnes and follow their course. 



The throat or hinder ])art of the mouth chamber of 

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



