210 AMPHIOXUS. 



2. The skin of the ventral surface is thrown into deep 



folds to allow for the distension of the atrial cavity 

 when the reproductive organs attain their full size. 

 Its general characters are the same as in A, 



3. The skeletal system is the same as in A, except that the 



branchial skeleton is present, and that there is no 

 buccal skeleton. 



4. The muscular system. 



a. The myotomes are similar to those of section A, 



but considerably larger. 



b. The ventral muscles form a thin sheet in the atrial 



fold of each side, extending from the lowest 

 myotome to the mid-ventral line, where the 

 muscles of the two sides meet in a median raphe. 



c. Small muscular bands are present in the suspen- 



sory folds of the pharynx and in the gill-arches. 



5. The pharynx is of an inverted heart shape in transverse 



section. 



a. The branchial bars, or gill-arches. Owing to their 

 obliquity and extreme closeness to each other, 

 twenty or more bars may be cut on each side in 

 a single section. Each bar is clothed on its 

 outer surface by the atrial epithelium, which is 

 a single layer of columnar cells. Within this is 

 the skeletal rod, deeply grooved along its inner 

 edge. From the rod a thin plate of connective 

 tissue extends inwards, clothed on each side by 

 the pharyngeal epithelium, a single layer of 

 very long columnar flagellate cells. 



The branchial bars are alternately large and 

 small, the difference being especially marked at 

 their outer edges. The large ones are the 

 primary, and the small ones the secondary bars. 



In the large primary bars a small space is 

 visible between the atrial epithelium and the 

 skeletal rod. This space, the branchial ccelomic 



