Section II—External Anatomy
The body of the lamprey is elongated and cylindrical, and covered by smooth, scaleless skin (Figure 2.2). The head extends posteriorly to include the slanted row of seven rounded or oval apertures, the external pharyngeal slits, which lie posterior to each of the laterally placed, lidless eyes.
The eyes, of moderate size, are covered by transparent skin. In preservatives, however, this skin turns opaque. Anteriorly the oral funnel forms a wide, sucking disk that attaches to the body of the lamprey’s prey (Figures 2.1, 2.3). The funnel’s margin
FIGURE 2.3 Anterior view of buccal funnel of the lamprey showing mouth and oral disk with teeth.
bears small, soft projections, the buccal papillae, which are primarily sensory structures. The funnel’s interior surface is termed the oral disk, and it is lined with numerous horny teeth, which are cornified epidermal derivatives and thus not homologous with the teeth of more derived vertebrates. Remove a tooth with forceps
FIGURE 2.4 Ventral view of the lamprey, with pleuroperitoneal cavity exposed (top). Detail of anterior end of pleuroperitoneal cavity and contained structures (left). Detail of cloacal region (right).
to observe a replacement tooth immediately underneath it. At the pit of the funnel lies the rasping tongue, which also bears horny teeth and, like them, is not homologous with the structure of the same name in more derived vertebrates. The tongue is used to abrade the skin of prey, so that its blood and body fluids may be ingested. Dorsal to the tongue is the mouth. The lamprey, being an “agnathan,” lacks jaws.
A single, median naris lies middorsally between the eyes (Figures 2.1, 2.8). A lighter patch of skin immediately posterior to the nostril denotes the position of the pineal complex, which lies just below the skin and functions as a photoreceptor that detects changes in light.
A lateral line system, which functions in detecting vibrations in the water, is present in the lamprey, but not typically conspicuous. Usually, however, pores for the canals are noticeable in the head region. The pores are arranged in rows. One occurs just posterior to each eye, and several others may be found between the eye and the anterior end of the oral funnel.The trunk extends from the head to the cloaca, a rather shallow midventral depression (Figure 2.4). The intestine opens into the cloaca through a slit-like anus. Posterior to the latter, there is a small urogenital papilla with a terminal genital pore through which gametes and excretory products leave the body. The tail extends posteriorly to the tip of the body and becomes laterally compressed. The lamprey has no trace of paired fins, but there are three median fins (Figure 2.2), the anterior dorsal and posterior dorsal fins and the caudal fin. The fins are supported by fin rays, which can be seen if the specimen is held against the light. Examine the lateral surface of the trunk or tail to observe the outline of the segmented series of muscular blocks, the myomeres. These are composed of longitudinal fibers extending between successive connective tissue partitions, the myosepta. Contraction of the myomeres on one side of the body flexes the body toward that side, and alternating contraction of the myomeres on opposite sides of the body produces the characteristic side-to-side swimming motion of fishes.