186 AMPHIOXUS 



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

 strong 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. 



8. 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 A, but con- 



siderably larger. 



b. The ventral muscles form a thin sheet running 



in the atrial fold of each side 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 suspensory 



folds of the pharynx and in the giU-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 one another 

 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. Immediately within this 

 is the skeletal rod, deeply grooved along its inner 

 surface. The rod is ensheathed in connective 

 tissue, which runs inwards from it as a median 

 plate clothed on each side by the pharjmgeal 

 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 ends. 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 



