38 THE SKATE 



from the junction of the supraorbital and the infraorbital 

 along the side of the vertebral column to the tip of the 

 tail; the hyomandibular canal with various branches. 

 Tubules lead to the surface from all of these canals and 

 may be easily seen in many specimens. 



C. The pit organs are on the dorsal surface and may 

 be seen with the unaided eye. They are in three groups : 

 a row of five or less under the eye ; a row along the back 

 on each side medial to the lateral line canal; and a group 

 medial to the spiracle on each side. 



2. The Organs of Special Sense. 



A. The Olfactory Organ. 



There are a pair of olfactory sacs on the ventral side of 

 the rostrum which open to the nostrils, but which have no 

 communication with the oral cavity. Expose the olfactory 

 sac by cutting away the skin just in front of the eye. 

 Then make a cross-section of the organ. Internally there 

 are numerous plates arranged in rows which are covered 

 with an epithelium sensitive to odors. 



Drawing 20. Draw the olfactory organ. 



B. The Eye. 



Expose the eye or eyeball, a spherical body, by carefully 

 dissecting away the skin and connective tissue from around 

 the eye. First cut away the skin over the eye, then care- 

 fully cut away a little of the cartilage between the eye and 

 the brain and a part in front of the eye. The eyeball is in 

 a cavity of the skull, the orbit, to which it is attached by 

 six eye muscles. Four of these muscles may be identified 

 from the dorsal side. Each muscle is attached at its two 

 ends and is more or less free in between. The attached 

 end which is fixed and immovable is called the origin while 



