Section IV—Sagittal Section

A sagittal section of the head and anterior part of the trunk reveals a number of interesting features, and, in particular, allows the observation of the extreme spe-

FIGURE 2.7 Cross section through body of the lamprey, seen in anterior view.

cializations of the lamprey’s digestive and respiratory systems. Using a large, sharp scalpel, make a clean sagit­tal section through the head of the lamprey. Maintain your cut as close to the sagittal plane as possible and extend the section about 10 cm posterior to the level of the last external pharyngeal opening. Compare your specimen to Figure 2.8. If your section is not quite sagit­tal, use a new scalpel blade and carefully shave tissue from the larger half until you reach midsagittally.

Examine the anterior part of the head to reidentify structures already noted, such as the annular cartilage, oral funnel, horny teeth, mouth, and tongue. Trace pos­teriorly from the mouth, over the anterodorsal surface of the tongue, into the oral cavity, which has a short anterodorsal extension, the oral cecum. Posteriorly the oral cavity passes into the “esophagus” dorsally and the respiratory tube or “pharynx” ventrally. The terms set in quotation marks denote subdivisions of the embry­onic pharynx, and so are not homologous with the esophagus and pharynx of other vertebrates (although, of course the “pharynx” is partly homologous with the pharynx). This anatomical condition in the lamprey reflects its highly specialized feeding mode. It would make little sense to feed on fluids if they could easily escape through pharyngeal openings. During metamor­phosis from the larval stage, a horizontal partition develops that subdivides the originally single pharynx in dorsal and posterior portions.

The tongue is supported by the large lingual cartilage. Note the complex of muscles arranged around the lingual cartilage. This musculature is responsible for working the tongue. Protractor muscles extend anteri­orly ventral to the cartilage, whereas the retractor muscles extend posteriorly from it.

The small brain lies dorsally. It is not particularly similar to that of more derived vertebrates, but it is tripartite, having the same major subdivisions. Trace it posteriorly as it passes into the spinal cord. Note the large noto­chord, which begins ventral to the posterior half of the brain and extends posteriorly, ventral to the spinal cord, for the rest of the lamprey. Note the slender sections of the cartilaginous elements that contribute to the chon­drocranium. There are not true vertebrae, but cartilagi­nous blocks termed arcualia, on either side of the spinal

FIGURE 2.8 Schematic sagittal section through head and pharyngeal region of the lamprey. The vessels, esoph­agus, respiratory tube, and parts of the heart are shown with portions cut away, rather than in sagittal section.

cord. As noted above, however, these are difficult to find, and are lost in most prepared specimens.

Locate the naris. It opens into a short tube that leads to the dark-walled olfactory sac, directly anterior to the brain. The sac’s interior surface has numerous folds to increase surface area. The tube continues past the opening of the olfactory sac as the hypophyseal pouch, which ends blindly ventral to the brain and anterior end of the notochord. Dorsal to the nasal sac and anterior end of the brain is the pineal eye complex, a region spe­cialized primarily for light detection.

The heart lies in the pericardial cavity, posterior to the respiratory tube. Posteriorly it is separated from the pleuroperitoneal cavity by the transverse septum, which is stiffened by the pericardial cartilage. The heart has three chambers, in contrast to the four present in more derived vertebrates. These are, in order of blood flow, the sinus venosus, atrium, and ventricle. The spatial positions of these chambers do not follow the sequen­tial posterior-to-anterior progression typical of the heart of most fishes. Instead, the sinus venosus is a tubular structure, oriented dorsoventrally, that lies between the atrium and ventricle. The atrium mainly occupies the left side of the pericardial cavity and the ventricle mainly occupies the right side. The depiction of the heart in sagittal section (Figure 2.8) is somewhat idealized, as structures such as the intestine and inferior jugular vein are shown in their entirety, rather than as sectioned, as they would be in a true sagittal section. Compare these structures with Figure 2.9, which shows the relation­ships of these and other structures in transverse view.

The flow of blood passes forward from the ventricle through the ventral aorta, which sends out seven affer­ent branchial arteries to the capillaries in the septa between the pharyngeal slits. The blood is recollected by the efferent branchial arteries, which channel it into the dorsal aorta for distribution to the body. It is easiest to observe the dorsal aorta in a transverse section.

Blood from the head returns to the heart mainly through the paired anterior cardinal veins dorsally and the median inferior jugular vein ventrally. Much of the pos­terior part of the body is drained by the paired poste­rior cardinal veins, as well as the hepatic portal vein. The latter is difficult to identify and not considered further here except to note that it comprises a system of veins that drains blood from the intestine and conducts it to the heart. Dorsal to the heart, the right anterior

FIGURE 2.9 Cross section through body of the lamprey to show structure of the heart. A shows the anterior portion of the section in posterior view. B shows the posterior section in anterior view. Interpretative illustra­tions are below each photograph.

and posterior cardinal veins join to form a right common cardinal vein, which then enters the dorsal end of the sinus venosus (Figure 2.9). The left anterior and posterior cardinal veins join the right common cardinal vein. Note that this condition is distinct from that present in most jawed fishes, such as the shark, where the anterior and posterior cardinal veins of each side of the body unite to form their own common cardinal vein that enters the sinus venosus separately. The inferior jugular vein is a median structure and enters the ventral end of the sinus venosus. For most of its length, it is a median structure, but near the sinus venosus it veers slightly to the left.

In the tail, the caudal vein accompanies the caudal artery. The lamprey does not have a renal portal system, as do jawed fishes, so the caudal vein does not bifurcate into renal portal veins that then enter the kidneys. Instead the caudal vein bifurcates into the right and left posterior cardinal veins (Figure 2.7), which receive blood from the kidneys.

Key Terms: Lamprey

afferent branchial arteries

annular cartilage anterior cardinal veins anterior dorsal fin anus

archinephric duct arcualia

atrium

brain branchial basket buccal papillae caudal artery caudal fin caudal vein chondrocranium

cloaca coelom dorsal aorta efferent branchial arteries

“esophagus” external pharyngeal

slits

eyes fin rays genital ducts genital pore gonad head heart hepatic portal vein horny teeth hypophyseal pouch

(nasohypophyseal pouch)

inferior jugular vein intestine kidney lateral line system lingual cartilage (piston

cartilage)

liver mouth

myomere myoseptum (plur., myosepta) naris (nostril) notochord olfactory sac oral cecum oral funnel ovary pericardial cartilage pineal eye complex pleuroperitoneal cavity posterior cardinal veins posterior dorsal fin respiratory tube

(“pharynx”) sinus venosus spinal cord tail testis tongue (piston) transverse septum typhlosole urogenital papilla velum ventral aorta ventricle