70 COLOUR, FORM, ANt> MOVEMENT. 



Muscles. The proportion of muscle to the total body 

 weight in a fish is considerable. The amount of space 

 occupied may be demonstrated by pointing out that if we 

 except the cavity on the lower side of the body containing 

 the viscera, and the backbone and fins, practically all be- 

 hind the head consists of muscle. The outline of these 

 muscles may be seen through the skin in some fishes, e.g. 

 the haddock. That fishes are capable of long-continued 

 muscular effort we know from what has been learned 

 regarding their migrations ; for example, those of the eel 

 (see p. 73). 



Fins. The fish should be carefully and patiently watched, 

 and an endeavour made to interpret the action of the 

 several fins. To begin with let us enumerate these. Dis- 

 tinguish median fins in the middle line of the body, dorsal, 

 ventral, and tail. Fill these in upon your drawing. The 



df* 



Fig. 11. Diagram illustrating the externals of a Fish (Haddock). 

 na nasal pit; b, barbule ; op., operculum or gill cover; br. m., branchial 

 membrane, lower part of gill cover ; p.f., pectoral fin ; pv t f., pelvic fin ; u.ff.a., body 

 apertures ; d/." 2tn , dorsal fins 1 . 2 . 3 ; a./. "S ventral fins 1 and 2 ; <?/., tail-fin. 



number, size, and position of dorsal and ventral fins vary 

 in different types of fish. (See description of a fish, p. 77.) 

 There are also two sets of paired fins. These are compar- 

 able to the two pairs of limbs in higher animals. 



Consider the dorsal and ventral fins. We recall the un- 

 stable equilibrium of the fish and note that these fins will 

 help to maintain an even balance. As we watch the fish 



