. Chordate morphology. Morphology (Animals); Chordata. dorsal ftn nasohypophyseal opening buccal funnel caudal fin nasohypophyseal opening ,' tentacles branchial opening PETROMYZON OR LAMPETRA eye phoryngocutaneous duct {on left side^ EPTATRETUS opening of slime gland*"^ bronchial opening y-°-^-. Figure 2-1. Some external features of the lamprey, top, and hagfish, bottom. (Figure 2-1), is at the tip of the snout in the hagfish. There are three pairs of tentacles around the nasohypophyseal opening and the mouth. The mouth is circular, but not distinctly funnel-shaped, and it lacks horny te
. Chordate morphology. Morphology (Animals); Chordata. dorsal ftn nasohypophyseal opening buccal funnel caudal fin nasohypophyseal opening ,' tentacles branchial opening PETROMYZON OR LAMPETRA eye phoryngocutaneous duct {on left side^ EPTATRETUS opening of slime gland*"^ bronchial opening y-°-^-. Figure 2-1. Some external features of the lamprey, top, and hagfish, bottom. (Figure 2-1), is at the tip of the snout in the hagfish. There are three pairs of tentacles around the nasohypophyseal opening and the mouth. The mouth is circular, but not distinctly funnel-shaped, and it lacks horny teeth. The body IS smooth and in life is covered with sHme produced in part by the slime glands, whose openings can be seen forming a line along the ventrolateral aspect of the body. The eyes are indistinct in the case of Eptatretus, and they cannot be seen in Myxine. There are 10 to 15 pairs of bran- chial openings in Eptatretus, the last on the left side serving also as an opening for a passage from the pharynx known as the pharyngocutaneous duct. to the branchial openings are pores of slime glands. Mvxine has only a single pair of openings since all of the gill pouches (5 to 7 pairs) open together. The left opening also serves the pharyngo- cutaneous duct. The caudal (tail) fin extends anteriorly above and below the caudal lobe of the body. The cloaca is a slit-like opening, splitting the anterior margin of the caudal fin (or anal part of the caudal fin). Anterior to the cloacal aperture, a median fin extends to the posterior branchial openings. Chondrichthyes The sharks and their relatives are placed in this group of gnathostomes on the basis of their cartilaginous skeletons. In terms of their external anatomy, this group has a skin covered by small, spine-bearing scales (placoid scales); some- times there are larger spine-bearing plates scattered over the body (ray); in some the skin is naked. The holocephalan differs most markedly from the elasmobranchs in having the
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