Lamprey Dissection / Vertebrate Biology

Background:

The vertebrates constitute the largest of the four chordate subphlya. A vertebral column (which, in the lamprey, is present as fragments), and a well-defined head. The earliest vertebrates were the jawless fishes that are placed in the class Agnatha. All living agnathans belong to the subclass Cyclostomata, or round-mouthed fishes, which consist of the lampreys and hag-fishes and are the most primitive group of living vertebrate. It has been suggested that cyclostomes are descendants of an extinct subclass of jawless, fish-like creatures, the ostracoderms, which are the oldest and most primitive fossil vertebrates known, having vanished about 350 million years ago. However, although the cyclostomes do exemplify a primitive stage of development, they are no ancestral vertebrates.

All eight genera of lampreys belong to the order Petromyzoniformes. They may be marine or freshwater and are found throughout the temperate parts of the world: in the oceans, along the coasts, or in large inland lakes. The usual form studied in the laboratory is the marine lamprey Petromyzon Marinus, whose relatively large size enables detailed anatomical observation of both its primitive and its specialized characteristics. Its primitive characteristics include the absence of jaws, true teeth, and paired appendages, and the presence of a continuous notochord, segmental musculature, a straight alimentary tract, and a poorly developed brain, skeleton, and reproductive system. Its specialized characteristics are its eel-like shape, a head without “bony armor,” and its mode of feeding.

The lamprey attaches itself by the mouth cup to other fishes, rasps at its preys flesh with its horny tongue, and then sucks its blood. The digestive and respiratory systems are modified to conform to this parasitic habit, and the lamprey is able to secrete an anticoagulant that permits continued utilization of the host's blood.

The marine lamprey has become adapted to a freshwater habitat. Thus, many spend the entire life cycle in the Great Lakes, where they cause intense depletion of the natural fish population.

External Morphology:

Head – Cylindrical in shape, it extends posteriorly as far as the last external gill slit on each side.

Trunk – Cylindrical, but it becomes laterally compressed near the tail region and extends from the end of the head to the cloacal aperture.

Tail – Extends posteriorly from the cloacal aperture.

Fins – Three median fins, the anterior dorsal fin, the posterior dorsal fin, and the caudal fin, are present. All are supported by the cartilaginous fin rays.

Buccal Funnel – Supported by an internal ring of cartilage, this hood-shaped structure is a sucking disk that attaches securely to the host. Its opening is fringed with numerous finger-like structures, the buccal papillae, and its interior studded with rows of brown horny teeth.

Nostril – Single median mid-dorsal opening located at the top of the head. It is the external aperture of the olfactory apparatus.

Pineal organ – The pineal organ lies just beneath the surface of the skin, and its position is demarcated by the small oval are located just posterior to the nostril. It detects changes in light and initiates diurnal color changes in the pigmentation of the skin.

Eyes – Lidless, functional organs for sight. There is one on each side of the head.

Lateral Line System – Consists of groups of pores that are sense organs that detect vibrations and movements in the water. Groups of such pores may be seen (with the aid of a hand lens) extending caudally from the top of the lateral eye, dorsolaterally beneath the eye, and caudally on the ventral surface from the buccal funnel. This receptor system is probably sensitive to changes in the current and turbulence of the water around the animal.

External Gill Slits – Seven oval apertures behind the eye on each side of the head.

Myomeres – Muscle segments whose outline may be seen through the skin of the trunk and tail.

Cloacal aperture – The opening of the shallow pit on the underside of the body at the posterior end of the trunk. The urogenital papilla, through which the excretory and genital products are emitted, may protrude from the aperture. The anus, a slit-like opening of the intestine, is just anterior to the papilla.

Sagittal Section:

Myomeres – Each consists of bundles of longitudinal muscle fibers that attach onto the connective tissue myosepta that separate the myomeres.

Brain – The lobulated structure above the anterior end of the notochord. It extends into the neural canal as the spinal cord. The adenohypophysis has three parts seen in the freshly cut specimens.

Pineal organ – Shaped like an inverted pyramid, it lies above the brain.

Nostril – This external aperture leads into the narrow canal that opens into the expanded olfactory sac. The canal continues caudoventrally as the elongated nasohypophyseal pouch, which ends blindly.

Cranial cartilages – Two median cartilages, anterior dorsal, and posterior dorsal, are present.

Notochord – This skeletal axis beneath the spinal cord supports the body during myomere contraction.

Dorsal aorta – The thin tube beneath the notochord.

Buccal funnel – Lined with rows of horny teeth, if had a supporting annular cartilage that can be seen in section at both ends of the buccal funnel.

Mouth – The aperture at the back of the buccal funnel dorsal to the tongue. The tongue, too, is covered with horny (lingual) teeth.

Pharynx – The chamber sloping caudoventrally from the mouth. A cavity, the hydrosinus, extends anteriorly from the dorsal pharyngeal wall. The floor of the pharynx is supported by the long median lingual cartilage and the well-developed lingual muscle.

Esophagus – This, the first part of the digestive tract, is a thick-walled tube that is the dorsal continuation of the pharynx. It extends beneath the dorsal aorta until it expands somewhat to form the intestine.

Branchial tube – The ventral continuation of the pharynx, this tube lies beneath the esophagus and ends blindly. Its opening is guarded by a curtain-like velum. Its walls contain seven internal gill slits on each side. This tube is known also as the respiratory tube because its gill slits lead into gill pouches.

Ventral aorta – This blood vessel extends from the ventricle cranially beneath the brachial tube.

Inferior jugular vein – A median blood vessel that lies beneath the pharynx and extends caudally to open up into the sinus venous of the heart.

Branchial basket cartilages – These can be seen in section beneath the inferior jugular vein. They actually extend laterally and upward within the body wall, which they support.

Cross Sections:

Section through the eyes:

Pharynx – The central cavity is surrounded by four masses of pharyngeal muscles.

Eyes – Located on either side of the pharynx, each consists of two parts: a large lens and a cup-shaped retina, made up of neural and pigmented layers. Ocular muscles extend medially from the retina.

Lingual cartilage – The median cartilage beneath the pharynx. On either side of the cartilage are lingual muscles.

Pharyngeal glands – Embedded in the lingual muscles on the ventrolateral sides of the pharynx, they secrete an anticoagulant that keeps the blood of the host animal from clotting. Their inconspicuous ducts open up on the underside of the tongue.

Cranial cartilage – One of several such cartilages making up the chondrocranium. This cartilage supports the nasohypophyseal pouch, above which are the olfactory sac and the pineal organ.

Carotid arteries – Located on both sides of the chondrocranium, they supply the eyes and the brain.

Myomeres – Seen on either side of the brain.

Section Through Branchial Tube:

Spinal cord – The dorsalmost midline structure.

Notochord – The spherical mass directly beneath the spinal cord.

Anterior cardinal veins – Vessels located on either side of the notochord.

Dorsal aorta – The blood vessel that lies beneath the notochord.

Esophagus – This part of the digestive tract is characterized by the numerous folds of its inner lining that extend from its thick walls.

Branchial tube – It has a thin wall and is located immediately beneath the esophagus.

Gill pouches – These are respiratory sacs or internal gills, lined with gill lamellae, and are located on either side of the larynx.

Ventral aorta – An unpaired vessel posterior to the fourth gill pouch; anterior to the fourth gill pouch it is paired.

Lingual muscle – The prominent muscle mass beneath the ventral aorta.

Inferior jugular vein – The lowermost midline structure. At the anteriormost part of the gill region it divides into two jugular veins.

Section Through Trunk:

Posterior cardinal veins – Located on either side of the dorsal aorta.

Intestine – Located in the mid-ventral part of the body cavity.

Gonad – This is a large unpaired median organ located above the intestine. In the male the gonad is the testis, which consists of numerous leaf-like lobes. In the female it is the ovary, which consists of a mass of eggs.

Kidney – These are located on either side of the gonads. They are drained by large ducts that run the ventral border of the kidney.

Section Through Tail:

Fin rays – These support the fin.

Caudal artery – Small blood vessel lying beneath the notochord. Beneath the caudal artery is the caudal vein.