ZOOLOGY OF CHORDATES STUDY GUIDE - Zakharenko M.O. - 2015

CHAPTER 2. SUBPHYLUM VERTEBRATES, OR CRANIATES - VERTEBRATA, SEU CRANIATA

2.1. Division Agnatha or Entobranchiata

The division Agnatha is also called Entobranchiata, as the gills (gill pouches) of these animals are lined with folded endoderm. It unites fossil and modern vertebrates in which the notochord serves as the primary supporting axis of the body throughout life. The division includes two extinct classes (Pteraspidomorphi, Cephalaspidomorphi) and the modern Class of cyclostomes (Cyclostomata).

The Class Cyclostomata comprises 38-45 species of modern Jawless vertebrates, which are grouped into two subclasses: Lampreys (Petromyzones) and Hagfishes (Myxini). This is the most ancient class of living vertebrates. They are characterized by a cartilaginous Skeleton with poorly differentiated individual elements, primitive gills (5-16 pairs of endodermal gill pouches), a simply structured circulatory and Reproductive System (Gonads lack independent gonoducts), a small Brain size, etc. Lampreys live in seas, brackish waters, rivers, and streams; hagfishes live exclusively in seas (they perish at a Water salinity below 25 %o). They swim by undulating their bodies in a snake-like fashion. Most of them are parasites and predators. They feed on the soft Tissues, Blood, and Lymph of aquatic animals, primarily fish.

General characteristics of cyclostomes as exemplified by the lamprey.

The lamprey (Lampetra mariae) is a representative of the class Cyclostomata. It belongs to the most primitive forms of modern vertebrates and possesses A number of specialized features in both its external and internal Structure, which developed as adaptations to a semi-parasitic lifestyle. The cylindrical body of the lamprey lacks an exoskeleton (Fig. 19).

Fig. 19. External view of a lamprey

1 - oral (suction) funnel; 2 - single nostril; 3 - eye; 4 - external openings of gill pouches; 5 - lateral line Organs; 6 - anus; 7 - urogenital papilla; 8 - dorsal fins; 9 - caudal fin; 10 - myomeres; 11 — myosepta

Its Skin is naked, covered with mucus constantly secreted by the glandular Cells of the stratified epidermis. Paired fins, or metapleural

folds, are absent in the lamprey. Unpaired fins—the dorsal and caudal fins—are weakly developed and supported by soft cartilaginous rays, the pterygiophores. The Organ of attachment in the lamprey is a wide oral funnel, surrounded at its margins by a cutaneous fringe. On the walls of the oral funnel are horny "Teeth"—Connective Tissue papillae covered with a cornified epidermis. On the lower part of the funnel, they fuse into the lower horny plate, and on the upper part, into the upper horny plate. Deep within the oral funnel lies a muscular Tongue. The tongue plays an auxiliary role when the lamprey attaches to the host's body or other objects. On the HEAD of the lamprey, There is a series of small lateral line pores that function as cutaneous Sensory Organs. The single nostril, which is not connected to the Oral Cavity, ends blindly in a hypophyseal pouch. Weakly developed eyes are located on the sides of the head, followed by seven pairs of external gill openings. At the posterior end of the body, on the ventral side at the boundary between the trunk and the tail, is the anus, behind which the urogenital opening is located at the tip of a papilla. The internal Skeleton of the lamprey is characterized by connective tissue structures and Cartilage. The Axial Skeleton is represented by the notochord (Fig. 20).

Fig. 20. Skeleton of a lamprey: A - dorsal view; B - lateral view Neurocranium: 1 - braincase; 2 - olfactory capsule; 3 - auditory capsule;

4 - subocular arch. Splanchnocranium: 5 - annular cartilage; 6 - anterior dorsal cartilage; 7 - posterior dorsal cartilage; 8 - lateral cartilages;

9 - rod-like cartilages; 10 - hyoid cartilage; 11 - branchial basket; 12 - pericardial cartilage. Axial skeleton: 13 - notochord;

14 - rudiments of neural arches

The paired series of cartilages (double for each segment) embedded in this sheath represents the rudiments of the upper neural arches of the vertebrae. The Skull of the lamprey is very primitive. The neurocranium skeleton is represented by a cartilaginous basal plate underlying the brain. The auditory capsules adjoin it on the sides, and the cartilaginous olfactory capsule is at the front. The visceral skeleton is represented by the branchial basket, which is joined anteriorly by modified branchial arches—paired styloid and subocular cartilages. Behind this system of cartilages lies the cartilaginous pericardial sac. The branchial basket is a delicate cartilaginous lattice formed by nine vertical arches and four longitudinal cartilaginous commissures. The basis of the skeleton of the oral funnel's suction apparatus is the annular

Fig. 21. Cytology/practical/54.html">Longitudinal section of a lamprey's body

1 - annular cartilage; 2 - anterior dorsal cartilage; 3 - posterior dorsal cartilage; 4 - annular cartilage; 5 - hyoid cartilage; 6 - olfactory capsule;

7 - notochord; 8 - brain; 9 - Spinal Cord; 10 - nostril; 11 - hypophyseal pouch; 12 - teeth of the lingual plate; 13 - Esophagus;

14 - Liver; 15 - three anterior gill openings; 16 - atrium; 17 - ventricle of The Heart; 18 - respiratory tube; 19 - dorsal aorta;

20 - entry point of the cardinal Veins into the sinus venosus; 21 - hepatic vein; 22 - sinus venosus; 23 — Ovary

Digestive System. The digestive tract begins with the preoral funnel, which is more developed in lampreys than in hagfishes (Fig. 22).

Fig. 22. Oral funnel view of a lamprey: 1 - Mouth; 2 - tip of the tongue; 3 - horny tooth plate at the tip of the tongue;

4 - supraoral horny tooth plate; 5 - infraoral horny tooth plate; 6 - upper labial "teeth";

7 - lateral labial "teeth"; 8 - small marginal labial "teeth"; 9 - cutaneous fringe of the funnel margin

Along the margin of the oral funnel, there are small skin folds that facilitate better attachment. Hagfishes have two pairs of barbels around the mouth. Keratinized epithelial outgrowths on the inner surface of the funnel form horny teeth and tooth plates; their size, shape, and arrangement are of taxonomic importance.

Deep within the funnel lies a circular mouth, bounded below by the tip of a powerful tongue, which bears one or two large horny teeth or a complex horny tooth plate. Lampreys suck in small prey along with water. When attached to large prey, lampreys rasp through the skin using the tip of their tongue; hagfishes bite into soft tissues using their lingual teeth.

During metamorphosis of the lamprey larva, the lower part of the posterior pharyngeal wall expands, dividing the Pharynx into two isolated compartments: the esophagus and the respiratory tube (Fig. 23).

Fig. 23. General diagram of the lamprey pharynx structure:

A - larva, B - adult; 1 - pharynx; 2 - esophagus; 3 - respiratory tube;

4 - fold dividing the pharynx into the esophagus and the respiratory tube; 5 - oral cavity

The internal openings of the gill pouches open into the respiratory tube. From the oral cavity side, the entrance to the respiratory tube is covered by a respiratory fold called the velum. In a swimming lamprey, the velum is deflected and closes the lumen of the esophagus; water passes through the mouth into the respiratory tube. Unlike other vertebrates, a narrow tube—the esophagus—extends from the anterior to the posterior part of the body in the lamprey. It bypasses The Heart and leads into the intestine, which is separated from the esophagus by a valve. The anterior, slightly dilated part of the intestine Functions as a Stomach, while the posterior part acts as a rectum. A high fold of the mucous membrane runs along the entire intestine, forming a spiral. This is the spiral valve, which increases the absorptive surface of the intestine and slows down the passage of food through it. As a result, complete Digestion of food occurs. Due to its semi-parasitic lifestyle, the lamprey's intestine is not differentiated into distinct compartments. It is represented by a straight tube. A large liver is located beneath The Stomach. The Pancreas is absent. An outgrowth of the anterior intestine—the liver—functions as an endocrine organ. The respiratory tube has seven pairs of lateral openings leading into the gill pouches, which communicate with the environment via seven pairs of external gill slits (Fig. 24).

Fig. 24. Respiratory system of hagfishes and lampreys:

1 - pharynx; 2 - gill pouches; 3, 4 - gill openings

During respiration in lampreys, water passes through the oral cavity into the respiratory tube, from which it enters the gill pouches through the internal gill openings, and then exits through the external gill openings. If a lamprey is attached to its prey by the oral funnel, water enters the gill pouches through the external gill openings during respiration and also exits through these same openings. The main feature of the lamprey's Circulatory system is a well-differentiated two-chambered heart consisting of three compartments: a thin-walled sinus venosus, an atrium, and a ventricle (Fig. 25).

Fig. 25. CIRCULATORY SYSTEM OF the lamprey

1 - ROOT of the dorsal aorta; 2 - efferent branchial Arteries; 3 - anterior cardinal (jugular) vein; 4 - dorsal aorta; 5 - posterior cardinal vein; 6 - intestinal artery; 7 - afferent branchial arteries; 8 - ventral aorta; 9 - gill slits; 10 - ventricle; 11 - atrium;

12 - sinus venosus; 13 - hepatic vein; 14 - subintestinal vein; 15 - caudal vein and artery; 16 - carotid artery

From the latter arises the ventral aorta, which forms a prominent dilation at its base—the bulbus arteriosus. The unpaired gonads occupy most of the Abdominal cavity: the Testis in males and the ovary in females. They lack specialized genital ducts, and Gametes are discharged in the same manner. Upon rupture of the gonad walls, the gametes are shed into the body cavity, and from there, they pass through genital pores located on the walls of the urogenital sinus into the water, where Fertilization occurs.

The excretory system is represented by mesonephric Kidneys located in the body cavity closer to the caudal region and situated on the dorsal side as two elongated bodies. The Ureters (Wolffian ducts) connect to the urogenital sinus, which opens externally via the urogenital pore.

Although primitive, the brain of lampreys comprises all five regions characteristic of all vertebrates: the Forebrain, Diencephalon, Midbrain, Cerebellum, and Medulla Oblongata. They lie in a single plane, without forming the flexures characteristic of highly organized vertebrates (Fig. 26).

Fig. 26. Brain of the lamprey

1 - forebrain; 2 - olfactory lobes; 3 - Olfactory nerve; 4 - diencephalon; 5 and 6 - right and left habenular ganglia;

7 - pineal organ (epiphysis); 8 - Optic nerve; 9 - infundibulum; 10 - optic lobes; 11 - opening in the roof of the midbrain;

12 - floor of the midbrain; 13 - Oculomotor nerve; 14 - Trigeminal nerve; 15 - Facial Nerve; 16 - auditory nerve;

17 - medulla oblongata; 18 - Rhomboid fossa; 19 - rudimentary cerebellum

Lamprey species identification is based on the configuration of horny structures in the buccal funnels (Fig. 27).

Fig. 27. Various lamprey species

1 - Caspian; 2 - sea; 3 - Ukrainian; 4 - river; 5 - Pacific; 6 — tridentate