198 VERTEBRATA 



gill arches are the gill slits permitting the water to pass 

 from the pharynx to the exterior. The flap covering 

 the gills externally is the operculwn. A narrow tube, the 

 gullet or esophagus, leads from the large pharynx to the 

 stomach, from which extends the somewhat tortuous intes- 

 tine to its opening at the root of the tail. The liver may 

 be recognized by its large size and red color. 



The intestine, as well as the other organs, is held in place 

 by a transparent membrane known as the mesentery. This 

 is a part of the peritoneum, which appears as a thin, glisten- 

 ing membrane lining the entire abdominal cavity and 

 closely enveloping most of the organs therein. The elon- 

 gated narrow white organ is the pancreas, adjacent to which 

 there is in most species a small dark red body, the spleen. 

 The reproductive organs lying dorsal of the stomach are 

 white in the male and yellowish in the female. The air 

 bladder with a wall as thin as paper is of a grayish-white 

 color, and lies against the spinal column. To see the 

 kidneys, one must look carefully for a pair of flattened 

 brown bodies lying closely against the vertebral column 

 in the dorsal side of the body cavity. 



With a heavy knife the top of the skull may be removed, 

 and the parts of the brain with several of the ten pairs of 

 nerves issuing from its ventral aspect noted. By cutting 

 away the roof of the spinal column, the spinal cord, and 

 each nerve connected with it by two roots may be observed. 

 The eye cut into with a sharp knife reveals three coats: 

 an outer tough one, the sclerotic; a thin black one, the 

 choroid; and an inner pinkish one, the retina. The almost 

 round crystalline lens lies between the watery or aqueous 

 humor contained in the front chamber of the eye and the 



