SECTION THROUaH BUCCAL CAVITY 183 



transverse lines, 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 lines. 



Bound the margin of the notochord, and 

 especially at its dorsal surface, cells may be seen 

 which have undergone comparatively little modi- 

 fication. 



b. 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 

 myotomes from one another, and are continuous 

 at their outer borders with the dermis. 



c. The buccal skeleton. Sections rff the segmented 



rods which strengthen the lips are seen bordering 

 the entrance to the buccal cavity : a smaller rod 

 occupies the axis of each tentacle. In structure 

 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 somewhat cubical masses 



of muscle, lying along the sides of the body in 

 the compartments formed by the septa. Owing 

 to the obliquity of the myotomes, three or more 

 are cut in each transverse section, and owing to 

 their > shape each myotome is cut twice. 

 The myotomes of the two sides are not opposite, 

 but alternate with each other. 



b. Other much smaller muscles are seen in connection 



with the buccal skeleton and tentacles. 



5. The atrial cavity. 



The buccal cavity is, strictly speaking, part of 

 the atrial cavity, and not of the alimentary canal, 



