Chordata P, Vertebrata sP, Agnatha C, Petromyzontida O, Petromyzontidae F
Lamprey eels belong to Agnatha, the oldest known taxon of living vertebrates. Of a handful of living lampreys, the parasitic marine lamprey, Petromyzon marinus, is the most common and most readily available for laboratory use, although it is not the best example of its taxon. Agnaths are vertebrates with no paired appendages, no jaws, no cranium and no vertebrae. The skeleton is cartilaginous and the notochord persists through life. The larva is the ammocoetes, which closely resembles cephalochordates such as Branchiostoma (amphioxus). The ammocoetes (AM oh sete ease) is the vertebrate most like the most vertebrate-like invertebrate. As a vertebrate with striking similarities to a non-vertebrate it is an important evolutionary link supporting the hypothesized close relationship between protochordates and vertebrates.
The larvae live in freshwater streams, burrow in sediments, and are filter feeders utilizing phytoplankton. The larvae live two to six years before metamorphosing into parasitic adults.
Examine your specimen with your dissecting microscope. Like amphioxus, it is fishlike, laterally compressed, and fusiform. Anteriorly there is a poorly defined head with an oral hood enclosing a pre-oral space known as the vestibule. There is a long dorsal fin and the tail bears a caudal fin.
The nerve cord is a conspicuous, longitudinal, pink (stained) tube lying dorsally in the animal. Anteriorly the cord is expanded to form a distinct brain, a feature absent in amphioxus. The brain narrows abruptly to become the nerve cord. The external opening of the pouch is the nostril, on the dorsal midline.
Lampreys are monorhine agnaths which are unusual among vertebrates in possessing of a single olfactory capsule rather than the more typical pair of capsules. This single capsule is located on the posterior wall of the nasohypophyseal pouch.
Lampreys, like many early vertebrates have three eyes consisting of a pair of lateral eyes on the sides of the head and a single, unpaired median eye on top of the head. Each lateral eye is a spherical black structure lying beside the posterior brain.
The inner ear (otic vesicle) is a sense organ and can be seen as a large clear oval beside the anterior brain. During embryonic development the olfactory, optic, and otic sense organs develop as invaginations of surface ectoderm.
The notochord is a long thick rod lying ventral to the neural tube. It extends from the level of the inner ear posteriorly to the tip of the tail. Note that the notochord extends anteriorly only as far as the brain. Compare its length with that of the notochord of amphioxus. The name “cephalochordate” for amphioxus alludes to the presence of the notochord in the entire length of the head.
The posterior wall of the vestibule is a muscular partition, the velum. The gut begins with the mouth which penetrates the velum under the oral hood. The mouth is surrounded by small buccal cirri and cannot be seen in lateral view. Posterior to the mouth are visible the muscular lateral folds of the velum. Movements of the velum help move water through the mouth into the pharynx.
The pharynx with its seven visceral (= gill, = branchial) pouches is the region of the gut posterior to the mouth. Separating the pouches are eight visceral arches which bear gill lamellae on their anterior and posterior walls. Ammocoetes, like amphioxus, is a filter feeder, but it uses the velum, rather than cilia, to generate a flow of water that is the respiratory and feeding current. The ciliary current of amphioxus is chiefly for feeding and is not important in respiration. Also unlike amphioxus, the gills function both in feeding and in respiration.
During metamorphosis the lumen of the pharynx separates from the lumen of the gut and becomes a blind pouch. In adults it serves a strictly respiratory function, no longer being for filterfeeding by the parasitic adults. Food (the prey’s blood) flows posteriorly through the esophagus whereas a tidal respiratory current enters and leaves the gill pouches via the external gill slits. The mouth is permanently occluded by its attachment to the prey and cannot be used as an intake for the respiratory current.
The narrow esophagus leads posteriorly from the pharynx and soon widens to become the intestine. This portion of the gut extends posteriorly to open to the exterior via the anus. Use the dissecting scope to examine the gut contents.
Examine the area between the pharynx and intestine using the dissecting scope. Immediately posterior to the pharynx is the heart lying ventral to the esophagus. The liver lies just behind the heart and is also ventral to the esophagus. A large, easily seen, spherical, clear gall bladder lies near the liver. The gall bladder is lost during metamorphosis and is not present in the adult.
The body musculature is divided into overlapping myomeres. The muscle masses can be seen most clearly as a band running longitudinally just below the dorsal fin and above the nerve cord. They extend over the lateral aspects of the animal. Look carefully in the posterior region of the animal for the myosepta that separate successive myomeres.
Use the dissecting microscope to examine a large preserved specimen. Use microdissecting forceps to manipulate the lamprey as needed. Please handle them gently.
The body consists of a head, trunk, and tail. The head is small and poorly demarcated from the much longer trunk. It, the head, extends posteriorly from the pointed snout to about the level at which the body achieves its maximum diameter. At the anterior end of the head a large oral hood overhangs the spacious cavity known as the vestibule (Fig 1). During development the oral hood expands to form the walls of the adult buccal funnel. The inner roof of the vestibule is covered by patches buccal cirri (= oral papillae), which are easily seen with magnification. The posterior wall of the vestibule is the velum, a vertical, muscular, transverse partition covered by larger buccal cirri. The cirri play a role in selecting or rejecting food items by the filter feeding ammocoetes. They are both mechanical and sensory. The large mouth is an opening in the center of the velum. It is difficult to see because it is covered and obscured by the large buccal cirri. The mouth is not supported by jaws in these or any other agnath (= without jaws). The upper lip is the median dorsal portion of the oral hood. The lower lip extends as a low transverse ridge between the oral lobes of the hood.
The nostril is a conspicuous pore on the dorsal midline of the head. It is completely surrounded by a circular lip. Note that there is only one nostril in these monorhine (= one nose) agnaths. The nostril opens into the nasohypophyseal pouch which extends deep into the head and curves posteriorly ventral to anterior brain (prosencephalon). The nasohypophyseal pouch is of evolutionary interest as the homolog of the pituitary gland (= hypophysis). On its posterior wall is a single olfactory sac bearing the highly folded olfactory epithelium responsible for distance chemoreception (= olfaction).
Anterior to the nasohypophyseal pore are two short dorso-lateral rows of small lateral line pores. These are openings of the lateral line system, the all important sensory system of fishes. Other lateral line pores can be seen posterior to the nostril and at other places, especially on the head. Two long dorso-lateral rows of widely spaced pores extend along the trunk, near the dorsal midline.
The two lateral eyes are on the sides of the head a little posterior to the level of the nostril but are deeply insunk into the head and are nonfunctional in the ammocoetes. You will not see them because they are deep in the opaque, uncleared tissue of the head. The unpaired third, or pineal, eye is on the dorsal midline immediately posterior to the nostril but is not discernable.
Most of the length of the body is the trunk. It extends from the anterior end of the widest part of the body posteriorly to the anus. The long dorsal fin extends along the dorsal midline for the entire length of the trunk (Fig 1). The dorsal fin is continuous with the caudal fin of the tail.
The swollen anterior portion of the trunk is the branchial region (= pharyngeal region). It contains the pharynx, seven pairs of gill pouches, seven pairs of external gill slits, and eight paired gill arches characteristic of lamprey larvae and adults. The seven external gill slits are easily seen forming a short line along both sides of the branchial region. Pharyngeal muscles, assist the velar muscles mentioned above, pump water into the mouth and pharynx (Fig 1). In amphioxus the feeding current is generated by cilia but in the ammocoetes muscles are used to generate a current that is both feeding and respiratory. Food particles in the incoming water remain in the pharynx and pass posteriorly into the intestine for digestion and absorption whereas the water passes laterally into an internal gill slit, then into a gill pouch where gas exchange occurs, and then out the external gill slit. The ciliary currents suitable for small animals are unable to create a water current sufficient to supply larger animals with food and oxygen. Substitution of muscular for ciliary pumps is one of the innovations that made it possible for vertebrates to become large.
The remainder of the trunk is smaller in diameter than the branchial region. The vent on the midventral line near the termination of the caudal fin marks the posterior limit of the trunk. The vent is the external opening of the cloacal pit. The intestine and kidney release their wastes into the cloaca. The vent is flanked by a pair of lips. Insert the microteasing needle between the lips to reveal the shallow cloacal pit. The anterior opening into the pit is the anus of the digestive system and the posterior opening is the urinary pore of the excretory system. The urinary pore sits atop a small urogenital papilla.
Observe the sides of the trunk (and tail) noting the segmentally arranged bands of axial myomeres (musculature) separated by connective tissue myosepta. The ventral midline of the trunk is marked by a longitudinal ridge, which shows no signs of segmentation. This is the transverse muscle (Fig 2).
Note the absence of paired appendages, there being neither pectoral nor pelvic fins. Jaws are also absent but that is not obvious externally. The absence of jaws and paired appendages are agnathan characteristics.
The tail is defined as the region of the body posterior to the anus (Fig 1). It bears the caudal fin on its midline, dorsal and ventral. The caudal fin is continuous dorsally with the dorsal fin but ventrally it ends a little posterior to the anus and the ventral trunk has no fin. The postanal tail is a chordate characteristic. The single median dorsal aorta is located ventral to the notochord. This large vessel drains oxygenated arterial blood from the gills and delivers it to the head and body.
Ventral to the pharynx is the large ventral aorta. Blood exits the heart via the ventral aorta which runs the length of the pharynx. Paired aortic arches exit the ventral aorta and extent through the gill pouches to the dorsal aorta. En route they break up into capillary beds in the gills where their blood is oxygenated. In some sections the connection between the ventral aorta (Fig 2) or the dorsal aorta may be visible.
Figure 2 Ammocoetes larva. Cross section through the pharynx at 50X.
Figure 1. Juvenile Branchiostoma, cleared specimen.
Identify the structures in amphioxus
Figure 2. Anterior end of a cleared juvenile Branchiostoma.