APPENDIX 575 



fish can conveniently be made by watching a goldfish. 

 Note : the respiratory movements, and the use of the tail 

 and fins. 



2. In side view, note : the same parts as in the dogfish (p. 573), 

 with which' make a comparison. Note also : the absence 

 of eyelids ; and of spiracle ; the gill cover, consisting of 

 the operculum proper and the expansible branchiostegal 

 membrane ; the double nostril on each side ; the separate 

 anal, genital, and urinary openings ; the scales (Fig. 273). 



3a. Lift the gill cover and observe the gills. 

 b. Dissect away the skin on one side of the body and note : 

 myomeres ; lateral line branch of vagus, starting under the 

 operculum ; the cutaneous branch of the trigeminal (peculiar 

 to Teleostomi), starting on the head and branching to supply 

 sense organs on the fins. 

 v. Open the mouth, examine the teeth. 



4. Make a flap of the right wall of the abdominal cavity by an 



incision passing in front of the anus, and turn this flap back. 

 Note : oesophagus, stomach, pyloric caeca, intestine, rectum ; 

 liver (three lobes), gall bladder, bile duct ; spleen ; air 

 bladder ; cceliac artery, portal vein, hepatic veins ; testis or 

 ovary, vas deferens or oviduct ; ureter. Pass a seeker into 

 genital duct and into ureter (Fig. 409). 



5. Open the air bladder. Note : vascular plexus from which gas 



is secreted into bladder ; dorsal aorta. Remove dorsal wall 

 of air bladder and note kidneys. 



flat plates of bone, completely covered by skin and overlapping backwards ; it is 

 shaped for cleaving the water. The mouth is at the front end ; in life it is 

 continually opening and taking in gulps of water, which passes into the pharynx 

 and outward on the sides of the head through four gill-clefts that open under a gill- 

 cover or operculum on each side. Above the mouth He the nostrils, which have no 

 internal openings. A shallow ventral depression in which lie the anal, genital, and 

 urinary openings is placed midway underneath. The widest part of the body 

 contains the body cavity, which reaches some way behind the vent. The narrower 

 hinder region or tail contains only muscle and backbone, and ends in two flukes, 

 which are used as a two-bladed screw in propelling the fish. The two pairs of limbs 

 or fins, used in steering, balancing, and backing, are quite unlike the corresponding 

 members of a frog or a man, and the pelvic or true hinder pair lies forward under 

 the throat. They and the dorsal and ventral fins by which the body is kept upright 

 are strengthened by rays. There is no neck. The body is stiffened by the back- 

 bone, composed of a row of bony rings or vertebrse ; which runs along the upper 

 side of its whole length except in the head, where it is continued by the skull. In 

 the middle region ribs arch outwards from the backbone in the body-wall. There 

 is no breastbone. As in all Vertebrata, the central nervous system lies in the 

 hollow of the skull and backbone, the body cavity below the latter, containing the 

 stomach, coiled intestine, and other viscera, and the heart in a part of the body 

 cavity known as the pericardial cavity, which here stands in front of the rest and is 

 cut off from it. From the heart, a ventral arterial system, consisting of a median 

 ventral aorta and an afferent branchial artery in each of the "branchial arches" 

 which separate the gill-clefts, conveys blood to the gills, which stand upon the 

 arches, whence it passes, oxygenated, to the body by means of a dorsal ^ arterial 

 system. In returning to the heart, most of it passes either through a hepatic portal 

 system in the liver or through a renal portal system in the kidneys. The latter are 

 dark brown strips of tissue lying under the backbone above the body cavity, from 

 which they are separated by a large, closed air-bladder. The genital ducts are 

 continuous with the gonads in both sexes, and have no connection with the 

 kidneys. 



38 



