THE PERCH 41 



neys. These organs are a pair of slender deep-red bands which 

 lie close against the dorsal body wall, one on each side of the 

 vertebral column. Their anterior ends unite just dorsal to the 

 oesophagus and form a large, dark-colored median mass called 

 the head kidney, which extends across the body cavity. Dissect 

 away the remains of the oesophagus and note the exact shape and 

 extent of the head kidney. The ureters are a pair of tubes which 

 run along the entire length of the medial borders of the kidneys, 

 joining at their hinder ends. The single vessel thus formed then 

 passes to the minute urinary pore just back of the genital pore ; 

 a small urinary bladder projects from the median portion of the 

 ureters. 



Exercise 8. Draw a sketch of the urinary system within an outline 

 of the body and the body cavity. 



The Nervous System. This consists of ( 1 ) the central nervous 

 system, which includes the brain and the spinal cord; (2) the 

 peripheral nervous system, which includes the paired cranial and 

 spinal nerves and the sympathetic nervous system; and (3) the 

 special sense organs. 



We shall study first the special sense organs ; they are the in- 

 tegumentary sense organs, the nasal organs, the eyes, and the ears. 



The integumentary sense organs consist of ( 1 ) minute scattered 

 sense buds, which occur principally on the head but also on the 

 body, and (2) the lateral line. This line is a straight canal in 

 the integument which extends along the side of the body from 

 the head to the caudal fin, with branches also upon the head. 



The nasal capsules are a pair of sacs situated in front of the eyes 

 and communicating with the outside by two nostrils on each side ; 

 no communication with the mouth is present. Cut off the outer 

 wall of one of the capsules and observe the delicate folds of the 

 sensory epithelium radiating from a central point. 



Exercise 9. Make a drawing of the capsule on a large scale. 



Cut away the folds of the capsule and observe the olfactory nerve 

 which enters it at its hinder side. 



The Eyes. Observe the central pupil of the eye, through which 

 light is admitted to the interior of it ; the iris, which surrounds the 



