Chap. II.] 



EXTERNAL CHARACTERS. 



filmy lower lids close up over the eye when thus withdrawn. 

 Round the dark oval pupil is a bright yellow iris. 

 Between the eyes is a minute Irow-spot (see p. 218). 



2. The Codfish. The cod (Gadus morrhua) lives in the tem- 

 perate and Arctic regions of the ocean. It is permanently 

 aquatic, and is hatched from ova which float at the surface of 

 the sea. Its form is admirably adapted for motion through the 

 water, the head passing into the trunk directly, without any 

 neck, and the trunk gradually tapering to the tail. Motion is 

 effected by the fins, of which there are ten six median, and two 

 pairs lateral. There are three median dorsal fins (d. i., d ii.,d. iii.), 

 (in the perch two, in the herring one) along the back, and two 

 median anal fins (a. i., a. ii.), (in the perch and the herring one) 

 on the ventral surface behind the anus. The powerful caudal 

 fin (c.) forms the main part of the tail. The paired fins are the 

 pectoral (pi.) on either side, just behind the head, and the pelvic 

 (pc.) or ventral fins, some- 

 what lower down and fur- 

 ther forward. These four 

 paired fins answer to the 

 four legs of the frog. The 

 pectoral fins are supported 

 on a shoulder-girdle, and P t-m pC " P 1 - 5-' ' ^' 



the pelvic On bones which FIG. 4. -Coo FISH: EXTERNAL CHARACTERS. 



probably represent the hip- a . Anal aperture, a. i. a. ii. Anal fins. b. Barbule. 



girdle. Note that the pel- br - m - Branchiostegal membrane, c. Caudal fin. 



. . i d. i. ii. iii. Dorsal fins. e. na. External nares. 



VIC fins, representing the ^ Genital aper t ure . I I Lateral line. op. Oper- 



hind-legs, are carried so far cuium. pc. Pelvic fin. pi. Pectoral fin. u. IM- 



f i i nary aperture. 



forward as to be anterior 



to the pectorals, representing the fore-legs. This is not so in all 

 fishes e.g. the salmon or the herring. The fins are supported 

 on bony fin-rays, which are, at the ends, soft and flexible. In 

 the perch the rays of the first dorsal fin are stronger, and are 

 produced upwards into sharp spikes. 



There are ten apertures, four median and three pair lateral. 

 The first median aperture is the mouth. This passes back into 

 the gullet ; but at the sides of the pharynx there are five gill- 



