164 BULLETIN OF THE BUREAU OF FISHERIES 
are well formed. While the mouth and pharynx show cubic or polygonal cells in 
two and three layers, this condition gives way to three layers of columnar epithelial 
cells in the region of the heart. Also, there is a very conspicuous sphincter of stri- 
ated muscle at this level. This muscle ends abruptly as we pass backward, the gut 
having an exceedingly thin muscular coat. 
Fig. 38.-42 millimeters Fig. 39. — 250 millimeters 
Descent of the mouth, division of the nasal pits, and appearance of the labial papillse, as seen in a series of anterior views 
of the developing sucker. 
At the anterior region of the liver, which is located just back of the heart, the 
epithelium changes”to the simple columnar type. The gut is a narrow tube while 
passing through this region, and is placed high in the body. It expands, however, 
before the posterior end of the liver is reached, and its lining mucous membrane is 
Fig. 36. — 16 millimeters 
Fig. 37. — 22 millimeters 
