comstock] goldfish AND GEOGRAPHY 277 



that it grows in layers. Extending along the sides of the body 

 from head to tail is a line of modified scales containing small tubes 

 connecting with nerves; this is called the "lateral line" and it is 

 believed that it is in some way connected with the fish's senses, 

 perhaps with the sense of hearing. 



Normally, the fish has seven fins, one along the back, called the 

 dorsal; one at the end of the tail, called the tail or caudal fin; one 

 beneath the rear end of the body, called the anal ; a "pair on the 

 lower side of the body, called the ventrals; and a pair just back 

 of the gill openings, called the pectorals. All these fins play their 

 own parts in the movements of the fish. The dorsal fin is usually 

 higher in front than behind and can be lifted or shut down like a 

 fan. This fin, when it is lifted, gi\'es the fish greater height and it 

 can be twisted to one side or the other and thus be made a factor in 

 steering. The anal fin, on the lower side, acts in a similar manner. 

 The tail fin is the propeller and sends the body forward by pressing 

 backv\'ard on the water, first on one side and then on the other, 

 being used like a scull. The tail fin varies in shape very much in 

 different species. In the goldfish it is fanlike, with a deeply 

 notched hind edge, but in some it is rounded or square. 



The paired fins correspond anatomically to our arms and legs, 

 the pectorals representing the arms, and the ventrals, the legs. 

 Fins are made up of rays, as the bony rods are called which support 

 the membrane; these rays are of two kinds, those which are soft, 

 flexible, many jointed and usually branched at the tip; and those 

 which are bony, not jointed and which are usually stiff spines. 

 When the spines are present in a fin, they precede the soft rays. 



In order to understand how the fish breathes, we must examine 

 its gills. In front, just above the entrance to the gullet, are several 

 bony ridges which bear two rows of pinkish fringes ; these are the 

 gill arches and the fringes are the gills. The gills are filled with tiny 

 bloodvessels, and as the water passes over them, the impurities of 

 the blood pass out through the thin skin of the gills and the life- 

 giving oxygen passes in. Since fish cannot make use of air unless 

 it is dissolved in water, it is very important that the water in the 

 aquarium jar should often be replenished. 



Lesson on a Goldfish 



Before the pupils begin the study, place the diagram shown on 

 the blackboard, with all the parts labelled. 



