XIII 



BACKBONED ANIMALS 



275 



unpaired fins on the back and tail and under-surface 

 arc fringes of skin supported by integumentary skeletal 

 rays or dermotrichia. 



Fishes are often resplendent in colours, which are 

 partly due to red, yellow, and black pigment contained 

 in branching amoeboid cells (chromatophores), and partly 

 to spangles of a silvery waste-product called guanin, 

 which are also contained in special cells (iridocytes). In 



FIG. 87. EXTERNAL FEATURES OF A COD (Gadus morrhna), A TYPICAL 



TELEOSTEAN FISH. 



As a practical exercise the student should verify the various features 

 on a specimen and contrast them with those of a skate. 



N, double nasal aperture on each side, of use only in connection with 

 the olfactory sense. Behind the nostril the lidless eye. Behind the eye, 

 deeply embedded in the skull, is the ear with no drum or external aper- 

 ture. 1)1, .D2, .D3, the unpaired dorsal fins, supported by integumentary 

 bony rays or dermotrichia. CF, the homocercal or secondarily symmetrical 

 caudal fin. VI, V2, the unpaired ventral fins, a, y, u, the anus, the 

 genital aperture, and the urinary aperture close together. LL, the 

 sensory lateral line. 



The posterior body behind a line drawn from a to D2 is solid muscle, 

 and is the locomotbr organ. The muscles are disposed in W-shaped 

 blocks or myotomes separated by connective tissue. 



B is a large tactile barbule hanging down below the front of the man- 

 dible or low r er jaw. GC, the gill-cover or operculum, supported by four 

 bones, covering the gill-chamber. Below it the branchiostegal membrane 

 supported by rays (BR). PC, the pectoral fin or fore-limb ; PV, the pelvic, 

 fin or hind-limb, which has been shunted into a jugular position. 



many cases fishes can change their coloration so as to 

 bring it into greater harmony with their surroundings. 

 Flat-fishes, like plaice, do this in many cases with great 

 rapidity, while one is looking at them. In other cases, 

 such as trout, the transformation is slow. The outer 

 skin or epidermis of fishes is transparent and so delicate 

 that it comes off on our fingers when we take hold of a 



