SECTION THEOUGH BTJCCAI, CAVITY 185 



the myotomes, separated from one'another by con- 

 nective-tissue septa. 



2. The skin consists of two layers. 



a. The epidermis is a single layer of short columnar 



cells, with a thick cuticle on their outer ends. 



b. The dermis is a very thin layer of connective tissue, 



underlying and supporting the epidermis. 



3. The skeletal system. 



a. The notocliord is slightly oval, its longer axis being 

 vertical. It is crossed by numerous wavy trans- 

 verse bands, the spaces between which are oc- 

 cupied by a homogeneous substance : this latter 

 is the coagulated semifluid matter occupying the 

 vacuoles of the notochordal cells, the outlines of 

 which are indicated by the wavy bands. 



Near the surface of the notochord, and 

 especially in the dorsal region, cells may be 

 seen which have undergone comparatively little 

 modification. 



h. The connective tissue forms a thick deeply- staining 

 investment to the notochord, which is prolonged 

 upwards to enclose the spinal cord, and outwards 

 to form the septa : these latter separate the myo- 

 tomes from one another, and are continuous at 

 their outer borders with the dermis. 



c. The buccal skeleton. Sections of the segmented 



rods which strengthen the lips are seen border- 

 ing the entrance to the buccal cavity : a smaller 

 rod occupies the axis of each tentacle. In struc- 

 ture these rods resemble the notochord. 



d. The skeleton of the dorsal fin. The dorsal fin is 



filled with a gelatinous homogeneous substance, 

 which stains slightly with reagents. 



4. The muscles. 



a. The myotomes are seen as squarish masses of 

 muscle, lying along the sides of the body in the 



