Lampreys are ancient vertebrates, offering a direct way to understand the body plan of some of the earliest vertebrates. Studying their anatomy through dissection reveals features that have persisted for millions of years, providing insights into the lineage of more complex animals. Examining both their external and internal structures illustrates their specialized adaptations and primitive characteristics.
Lamprey Characteristics
Lampreys belong to an ancient lineage of jawless vertebrates known as agnathans, distinguishing them from most modern fish. Their skeleton is composed of strong, flexible cartilage rather than bone, a primitive feature. These animals possess an elongated, eel-like body that lacks scales and paired fins. They have a distinctive round, sucker-like oral disc lined with rasping teeth, a single median nostril, and multiple gill pouches, often seen as seven external gill openings. Some species are parasitic, feeding on the blood and body fluids of other fish, while others have non-feeding adult stages.
External Anatomy
The most prominent external feature is the oral disc, a circular, sucker-like structure equipped with rows of teeth. Within this disc, a rasping tongue is visible, used for feeding in parasitic species. Behind the eyes, seven external gill pores are present, arranged in a line on each side of the body.
The lamprey has a single median nostril on its head. Eyes are present but lack eyelids. Along the sides of the body, a lateral line system detects water currents, pressure changes, and vibrations.
The body is characterized by an anterior dorsal fin, a posterior dorsal fin, and a caudal fin. These fins are median and lack supporting rays found in more advanced fish. The cloaca, an opening for the excretory and reproductive systems, is located near the base of the caudal fin.
Internal Anatomy
The digestive system begins with a short esophagus that leads directly into a simple, straight intestine; a true stomach is absent. The intestine contains a longitudinal fold called a typhlosole. A bilobed liver is present, surrounding the anterior part of the intestine, though a gall bladder is absent.
The circulatory system features a two-chambered heart, enclosed within a cartilaginous pericardium. This heart pumps blood through a ventral aorta, which branches into afferent branchial arteries supplying the gills. Oxygenated blood then collects into efferent branchial arteries before entering the dorsal aorta, distributing it throughout the body.
The nervous system includes a primitive brain with distinct regions, situated anterior to the notochord. Extending posteriorly from the brain is the dorsal spinal cord, which lies above the persistent notochord. The notochord serves as the primary axial skeletal support throughout the lamprey’s life. Cartilaginous structures called arcualia are present above the notochord, representing rudimentary vertebral elements.
The excretory system consists of a pair of mesonephric kidneys, which are the functional adult kidneys. These kidneys lead into a common mesonephric duct that opens into the urinogenital sinus. The reproductive system has a single gonad, either an ovary or a testis. Reproductive products exit through the cloaca.
Insights from Lamprey Dissection
Dissecting a lamprey offers insights into the ancestral vertebrate body plan. Lampreys represent an ancient lineage, with morphology largely unchanged over 360 million years, earning them the moniker “living fossils”. Their lack of jaws and paired fins, coupled with a cartilaginous skeleton and persistent notochord, provides a direct look at features present before the evolution of more complex vertebrate traits. The simple, straight digestive tract and the two-chambered heart demonstrate less specialized organ systems compared to jawed vertebrates.
Studying lamprey anatomy helps scientists understand the evolutionary progression of various organ systems. The presence of a dorsal hollow nerve cord and pharyngeal gill slits reinforces their position within the phylum Chordata, highlighting shared characteristics across all chordates. These primitive features serve as a “calibration point” for comparative research, informing understanding of how more complex structures developed in later vertebrate groups. This makes lampreys an important model for exploring the origins and diversification of the vertebrate subphylum.