The _mouth_ is a broad transverse slit upon the ventral surface of the head. The cartilaginous jaws can be seen and felt just within the mouth. Both upper and lower jaws are armed with rows of flat, sharply pointed teeth. Study their arrangement and approximate number. Jaws of previously dissected specimens should be examined under a dissection microscope. The exact form and arrangement of the functional teeth can then be ascertained easily, and an examination of the inner surface of either jaw will disclose several rows of developing teeth. As the young teeth develop they move, a row at a time, into position on the edge of the jaw; the oldest teeth, occupying the outer row, are shed at about the same time. The upper jaw is partly overhung by a lip-like fold of skin. At each side of the mouth is a pocket, directed obliquely, having no communication with the mouth. These _labial pockets_ provide places for the _labial cartilages_ (which can be felt along the medial edges) when the mouth is closed, and also afford freedom of motion to the mandible. Cut along the inner edge of the labial pocket and expose the cartilages for examination. (The labial pocket of Eugaleus is much smaller, and in front of the corner of the mouth rather than lateral to it. The two cartilages are completely separated from each other, the posterior one scarcely reaching the pocket.) In front of the mouth are the _nostrils_, their apertures apparently divided by projecting flaps of the anterior margin. Explore the cavity of the nostril with a probe to get a good idea of its size and form. Between the pelvic fins is the _cloaca_, a large depression into which open the alimentary canal, the excretory and genital ducts, and the abdominal pores. The opening of the alimentary canal, the _anus_, is at the anterior end of the cloaca. In preserved specimens part of the intestine is frequently everted through the anus. A large fleshy process, bearing a pore at its tip, projects from the dorsal wall of the cloaca. In the male this is the _uro-genital papilla_; in the female the _urinary papilla_. In the female a _genital pore_, the opening of the oviduct, is found on either side of the papilla. An _abdominal pore_, leading into the abdominal cavity, is found on each side of the cloaca at the posterior margin. These are frequently closed in young specimens. The cloaca of Eugaleus has a comparatively small opening upon the ventral surface, which must be enlarged before the parts described can be seen well. The laterally placed _eyes_ are without lids; observe the considerable difference in the amount of curvature of the dorsal and ventral margins of the eye. In Eugaleus there is a fold of skin stretched across the lower part of the eye which serves as an eye-lid, and corresponds to the so-called “third eye-lid” or nictitating membrane of other vertebrates.