I 4 o THE BOOK OF NATURE STUDY 



suited for burrowing in mud and creeping through holes, and 

 many other shapes. 



(6) The paired Fins, their relative position. Note in the 

 whiting, etc., that the hind (pelvic) fins are in front of the fore 

 (pectoral) fins ! There has been a strange shunting forwards. 



(c) The unpaired Fins dorsal, ventral, and caudal. Note the 

 supporting fin-rays; count them. Notice the power of raising 

 and lowering the dorsal and ventral fins. Distinguish hard 



n,a 



FIG. 48. The external character of the haddock Gadus aglefinus. There is a small 

 tactile barbule (b) below the front of the lower jaw ; the nostril (n.a.) has a double 

 opening on each side ; the gills are protected by a cover or operculum (op) supported 

 by four bones, and extended behind in a flexible membrane (br.tri) supported by bony 

 rays. The sensory lateral line is shown, and an anterior patch of dark pigment in the 

 skin. The pelvic fin (pv.f.), which is equivalent to a hind-leg, has come to lie in 

 front of the pectoral fin (p.f.) which is equivalent to an arm. Paired fins are limbs 

 without digits, and must be distinguished from unpaired fins, which are median exten- 

 sions of the skin supported by fin-rays. In this fish there are three unpaired fins 

 (dorsal fins) along the back (d.f\ df 2 , and d.f*\ two unpaired fins (anal fins) on the 

 ventral median line (a.f.? a.f?\ and a caudal fin (c.f.). There are three openings 

 close together in front of the first anal fin (a, g, u) (anal, genital, and urinary), the 

 openings of the food-canal, the genital ducts, and the ureters from the kidney. 



spine-like fin-rays from those which are not spine-like. Notice on 

 a dead stickleback the arrangement for locking the spines at the 

 base. Notice peculiarly modified fin-rays, e.g. the " fishing-rods " 

 on the back of the angler. 



(d) The Shape of the Tail. The apparently symmetrical type 

 seen in most fishes ; its stroke makes the fish go straight ahead 

 without necessarily rising or sinking. The markedly unsym- 

 metrical type of tail, seen in sharks, dog-fish, sturgeon, etc., 

 much more developed in its upper portion than in its lower ; 



