Lampreys are among the oldest lineages of vertebrates on Earth, and their anatomy reflects a body plan that diverged from the jawed-fish lineage hundreds of millions of years ago. When you open one up in a dissection tray, what you find inside is strikingly different from a typical bony fish: no jaws, no paired fins, no true bone, and a skeleton built entirely of cartilage. These features make lamprey dissection a staple of comparative anatomy courses, because the animal sits at a critical branch point in vertebrate evolution and lets you see what a vertebrate body looks like stripped down to its most ancient architecture.
External Features Worth Identifying First
Before making any incisions, the external anatomy of an adult lamprey offers a lot to examine. The body is elongated and eel-like, lacking the paired pectoral and pelvic fins you would find on a typical fish. Instead, the lamprey has one or two dorsal fins along the top of its body and a caudal fin at the tail, all supported by cartilaginous fin rays rather than bony spines. The skin is smooth and scaleless, covered in a layer of mucus-secreting cells.
Along each side of the head, posterior to the eye, sit seven external gill openings, or branchiopores. These round pores are one of the lamprey’s most recognizable external features and are easy to count during a dissection. Unlike bony fish, which draw water in through the mouth and push it over gills protected by an operculum, the lamprey’s gill pouches open independently to the outside. This is particularly important for parasitic species that feed while attached to a host: water can flow in and out of the gill openings even when the mouth is sealed against the host’s body.
The eyes are prominent in adult lampreys but lack true eyelids. Behind and slightly between the eyes sits a less obvious but anatomically important feature: the nasohypophyseal opening, a single median nostril on the top of the head. This opening leads to a blind-ended sac involved in olfaction. It is not connected to the pharynx, which is another departure from jawed vertebrates where the nasal passages typically communicate with the oral cavity.
The Oral Disc and Its Dentition
The most dramatic external feature is the oral disc, a funnel-shaped sucker at the anterior end of the body. In parasitic species, the disc is ringed with keratinized teeth arranged in a highly organized pattern. Terminology developed by researchers divides these structures into several groups based on their position relative to the central esophageal opening. Circumoral teeth form a ring directly around the opening itself. Intermediate teeth sit between the circumorals and the outer edge, and marginal teeth are the small structures at the very rim of the disc. A midline infraoral tooth sits below the esophageal opening, while a supraoral tooth sits above it.
Inside the disc, the tongue-like piston bears its own set of teeth. These tongue teeth are bilaterally symmetrical and consist of a transverse lingual lamina and a pair of longitudinal lingual laminae, each bearing multiple cusps.
The shape and arrangement of these structures differ among species and reflect their feeding strategies. In the sea lamprey, Petromyzon marinus, the lingual laminae have serrated edges suited for creating a small, deep wound through which blood is drawn. By contrast, species like Lampetra fluviatilis have a much larger central cusp on the transverse lingual lamina and a wider supraoral, adaptations for gouging and cutting out pieces of host tissue rather than simply lapping blood.
Respiratory Anatomy
Internally, the respiratory system is built around a series of gill pouches, typically seven on each side. Each pouch is lined with gill lamellae where gas exchange occurs. The pharynx in the adult lamprey is divided into a dorsal esophagus (leading to the gut) and a ventral respiratory tube, sometimes called the water tube or respiratory pharynx. This separation is critical for parasitic adults: it allows the animal to ventilate its gills by pumping water in and out through the branchiopores via muscular contractions, independent of whatever the mouth is doing.
When you dissect a lamprey and look at the gill region from the inside, you can see the individual pouches as distinct, rounded chambers connected to both the pharynx and the external gill openings. The cartilaginous branchial basket, a cage-like latticework of cartilage, supports these pouches and helps maintain their shape during the rhythmic compression and expansion that drives ventilation.
The Circulatory System
The lamprey heart sits in a pericardial cavity near the posterior end of the gill region. It consists of a sinus venosus, a single atrium, and a single ventricle, followed by a conus arteriosus that leads into the ventral aorta. Blood flows forward through the ventral aorta to the gills, where it picks up oxygen, and returns via dorsal aortic branches to the rest of the body.
The walls of the conus arteriosus and ventral aorta in the sea lamprey are notable for containing large concentrations of tubular microfibrils, roughly 11 to 17 nanometers in diameter, that function somewhat like elastic fibers. The lamprey’s conus also has lateral subendothelial distensions, structures not found in hagfish, which may play a role in smoothing pulsatile blood flow.
During dissection, the heart is relatively easy to locate. It sits ventral to the esophagus and posterior to the last gill pouch. The ventral aorta can be traced forward from the heart along the midline of the gill region. Lamprey blood is red, thanks to hemoglobin, and the circulatory system is closed, meaning blood stays within vessels rather than bathing the organs in open sinuses.
A Skeleton Without Bone
One of the most distinctive aspects of lamprey anatomy is the complete absence of bone. The entire skeleton is cartilaginous, and even the vertebral elements are rudimentary compared to jawed vertebrates. In adult lampreys, small arch-like cartilages sit above the notochord, which persists as the primary axial support throughout life. These vertebral elements, or arcualia, appear only in rudimentary form and are composed of unusual matrix molecules, including elastin-like proteins rather than the collagen-dominated matrix typical of most vertebrate cartilage.
Developmental research has shown that the vertebral elements in lamprey do not develop until metamorphosis, well after the fin cartilage has already formed in the larval stage. This late appearance contrasts sharply with jawed vertebrates, where vertebral development begins much earlier. The finding also raises questions about whether lamprey vertebrae are truly homologous to vertebrae in other vertebrates, or whether they evolved independently.
In a dissection, you will not see discrete, bony vertebrae when you open the body cavity. Instead, the notochord dominates the axial skeleton as a flexible, rod-like structure running the length of the body. The branchial basket in the gill region and the annular cartilage supporting the oral disc are among the most prominent cartilaginous structures you will encounter.
The Pineal Eye and Nervous System
Lampreys have long been valued in neurobiology because they are considered among the most basal extant vertebrates, making their nervous systems useful for understanding which features of vertebrate brains are ancient and which are derived. The brain is relatively simple in layout, with recognizable forebrain, midbrain, and hindbrain regions, and a spinal cord that runs the length of the body within the cartilaginous neural arches.
One structure that consistently surprises students during dissection is the pineal complex, visible on the dorsal surface of the head as a pale, slightly translucent patch of skin. In the sea lamprey, this complex consists of two subunits: a dorsomedial pineal organ and a ventral, left-lateral parapineal organ. Both contain photoreceptor cells capable of detecting light. The pineal organ itself has an end bulb, an atrium, and a nerve tract connecting it to the brain, and it houses two types of photoreceptor cells along with supporting and ganglion cells. Researchers have concluded that this organ functions as both a photosensory and photoneuroendocrine structure, translating light information into nervous and hormonal signals that influence behavior and physiology.
During dissection, the pineal region is best observed by carefully removing the skin and connective tissue from the top of the head. The translucent spot marking the pineal eye is usually visible externally before any cuts are made.
Digestive Tract and Associated Organs
The adult lamprey’s digestive system is relatively straightforward. The esophagus leads from the dorsal portion of the pharynx into a simple, straight intestine. There is no true stomach in most lamprey species, and the intestine has an internal fold called the typhlosole that increases the absorptive surface area without adding length. The liver is a prominent organ in the body cavity, usually brownish and easily identified during dissection. A gallbladder may or may not be present depending on the species and life stage.
The kidneys in the lamprey are of a type called opisthonephric, meaning they develop from the posterior portion of the embryonic kidney tissue. They run along the dorsal body wall as elongated, ribbon-like organs. The archinephric duct carries waste from the kidneys to a urogenital papilla near the cloaca. In a dissection, the kidneys can be found by looking along the dorsal roof of the body cavity after the digestive organs are reflected to one side.
How Parasitic Feeding Actually Works
Parasitic lamprey species attach to host fish using the suction of their oral disc, then rasp a hole in the host’s skin with the piston-like tongue tipped in keratinized denticles. What makes long-term feeding possible, sometimes lasting days on a single host, is the secretion of the buccal glands. These glands sit in the oral cavity, and when the tongue is retracted, they occupy much of the opening at the rear of the buccal space. Because the glands are not enclosed in musculature, their contents are likely discharged by mechanical pressure as the toothed piston moves forward.
The secretions are pharmacologically complex. Research using spatial metabolomics on lamprey buccal glands has revealed that parasitic lampreys must simultaneously suppress the host’s immune response (to avoid triggering defensive behavior), block pain signaling, and prevent blood clotting at the wound site. The glands produce proteins that act as anticoagulants, ion channel blockers, and immune suppressors, a chemical toolkit that enables the lamprey to feed undisturbed for extended periods.
In a dissection, the buccal glands can be found by opening the oral cavity and looking posterior to the teeth. They appear as soft, whitish tissue masses and are most prominent in adults of parasitic species. Non-parasitic lamprey species, which do not feed after metamorphosis, have reduced or absent buccal glands.
Larval Anatomy and the Ammocoete Stage
If you dissect a larval lamprey, known as an ammocoete, the anatomy looks remarkably different from the adult. Ammocoetes are filter feeders that live burrowed in the sediment of streams and rivers, and their body plan reflects that lifestyle. The oral disc and keratinized teeth are absent; instead, the larva has a simple oral hood. The eyes are small and covered by skin, not yet functional in the way adult eyes are. The gill region is present but the branchial apparatus is adapted for both respiration and filter feeding.
The most distinctive internal feature of the ammocoete is the endostyle, a glandular groove running along the floor of the pharynx. Ammocoetes feed by trapping small waterborne particles in mucus within the pharynx. The mucus that traps food particles originates from dense accumulations of goblet cells in the lateral parts of the pharynx, not from the endostyle itself, which is a correction to what older textbooks commonly stated. The endostyle does secrete digestive enzymes into the condensed food cord, but it is not the source of the feeding mucus.
The seven stages of metamorphosis from ammocoete to adult have been described in detail for the sea lamprey. External changes include the emergence of functional eyes (stages 1 through 3), development of the oral disc and its opening (stages 2 through 4), growth and cornification of teeth (stages 4 through 7), and changes in body coloration (stages 5 through 7). Internally, the transformation is equally dramatic.
The Endostyle-to-Thyroid Transformation
Perhaps the most evolutionarily significant anatomical feature revealed during lamprey dissection is what happens to the endostyle during metamorphosis. In the ammocoete, the endostyle binds iodine and produces thyroid-hormone precursors, functioning in ways that parallel thyroid tissue even though it is structurally a glandular groove open to the pharynx. Autoradiograph studies have shown that iodine binding in the endostyle is correlated with the production of a specific secretion that, in fully active cells, takes on a granule-like form.
During metamorphosis, a portion of the endostyle’s epithelial cells redifferentiate and transform into thyroid follicles, the same type of hormone-producing structures found in all other vertebrates, including humans. This transformation is considered one of the strongest pieces of evidence linking the vertebrate thyroid gland to the endostyle of non-vertebrate chordates like lancelets and tunicates. Molecular studies using markers such as thyroid transcription factor-1 have reinforced this homology, and researchers have found that a duct persists connecting the endostyle lumen to the pharynx throughout development, a structure that resembles the thyroglossal duct in mammalian thyroid development.
Recent work, however, has complicated the story. Analysis of the gene regulatory networks controlling endostyle and thyroid development suggests these programs may be more independent and modular than previously assumed. One hypothesis now being explored is that the endostyle in modern lampreys may have been secondarily acquired rather than directly retained from the common ancestor of all chordates. If this turns out to be correct, the lamprey endostyle would still be functionally analogous to the endostyle of lancelets and tunicates but would not be a simple holdover from a shared ancestor.
The Lamprey Immune System
For decades, the lamprey posed a puzzle for immunologists. The animal clearly mounts immune responses and possesses anatomical structures with lympho-hematopoietic characteristics, including regions in the gill area and intestine where immune cells concentrate. Yet researchers could not find the immunoglobulin or T-cell receptor genes that define the adaptive immune systems of jawed vertebrates. The resolution came with the discovery that lampreys use an entirely different system for generating immune receptor diversity: variable lymphocyte receptors, assembled through a rearrangement mechanism on a novel extended genetic locus.
In a dissection, you will not see discrete lymph nodes or a spleen in the mammalian sense, but tissue with immune function is distributed in the gill region and along the gut. The typhlosole of the intestine, in addition to its absorptive role, is one site where immune cells aggregate. Understanding that the lamprey achieves adaptive immunity through a completely different molecular toolkit than jawed vertebrates is one of the reasons the animal remains so important to comparative immunology.
Practical Notes on Preserved Specimens
If you are working with preserved lamprey specimens in a lab, it is worth knowing that the preservation process changes the animal’s appearance in measurable ways. Studies on larval lampreys preserved in formalin or in formalin followed by alcohol storage have shown that both pigmentation and body measurements are significantly affected. Morphometric lengths shrink, and pigmentation patterns shift, with nearly all changes occurring within the first three weeks of preservation. Different preservatives affect different characters to different degrees.
For dissection purposes, this means that colors you see on a preserved specimen do not accurately represent the living animal. Structures may also be slightly smaller or distorted compared to their fresh dimensions. If you are attempting to identify species based on pigmentation or body proportions, using measurements calibrated for the specific preservative method is important. Fresh or recently preserved specimens will give you the most accurate picture of external anatomy, while long-term preserved specimens are still perfectly adequate for identifying internal organs and structural relationships.
Why Lampreys Keep Showing Up in Research Labs
Lampreys have been subjects of neurobiological research since the 1830s, and their utility extends well beyond classroom dissection. One active area of study is spinal cord regeneration. Unlike mammals, lampreys can recover locomotor function after spinal cord injury, and researchers use the animal to investigate which cellular and molecular mechanisms make this possible. The relatively simple and well-mapped nervous system makes it feasible to trace individual neurons and their connections before and after injury.
The lamprey’s position as one of the most basal extant vertebrates means that features shared between lampreys and jawed vertebrates are likely to be ancestral to all vertebrates. Conversely, features present in jawed vertebrates but absent in lampreys may have evolved after the two lineages diverged. This comparative logic has informed research on everything from brain organization to the origins of the adaptive immune system, and it is ultimately the reason lamprey dissection remains a fixture of biology education rather than a historical curiosity.