Vertebrate Zoology: A Study Guide - T. A. Dauda 2014

Multicellular Animals
Chordates
Subphylum Vertebrata (Craniata) - Class Cyclostomata

General characteristics

Cyclostomes are the most primitive group of extant vertebrates. In their external appearance and certain biological traits, they resemble fish, but a whole range of features sets them apart as a distinct group of vertebrate animals.

Cyclostomes lack jaws and paired appendages, which underlines the primitive nature of their Organization. The olfactory capsule is unpaired and opens outward via a single nostril. The gill apparatus is represented by peculiar gill pouches.

An adaptive organ for a parasitic lifestyle is the oral disc (or funnel), at the bottom of which lies the Mouth opening. This characteristic gave the Class its name — cyclostomes (meaning "round-mouthed").

They are characterized by horny Teeth and naked Skin that is very rich in glands.

The majority of species in this class are specialized parasites and predators; they attach themselves to aquatic animals, predominantly fish.

The body shape of cyclostomes is eel-like or worm-like. The caudal fin is homocercal (diphycercal). Paired fins are absent. There are 1–2 dorsal fins, and females develop a small anal fin prior to spawning. In hagfish, dorsal fins are undeveloped.

Integument. The skin is soft, with no traces of an exoskeleton. The epidermis contains numerous unicellular mucous glands. The mucus serves a protective function, while in hagfish, it facilitates entry into the victim's body.

Locomotor System. The notochord persists throughout life. A Connective Tissue sheath — the "fat cushion" — encloses both the notochord and the Spinal Cord. In lampreys, small rod-like cartilages form within this sheath, representing rudimentary vertebrae. These are absent in hagfish.

The chondrocranium is at an evolutionary stage corresponding to the early embryonic development Stages of the Skull in other vertebrates. It surrounds the Brain only ventrally and laterally. The occipital region is undeveloped.

The visceral Skeleton is peculiar, comprising the Skeleton of the oral funnel, the branchial basket, and the pericardial Cartilage. The branchial basket is present only in lampreys and is absent in hagfish.

The caudal and dorsal fins are supported by long, thin cartilaginous rays.

The Muscular System is more powerful than that of acranians. It consists of myomeres separated by partitions known as myosepta. In the HEAD, a complex system of Muscles differentiates for the oral funnel, Tongue, and gill pouches.

Digestive System. The digestive tract begins with a funnel, which is more developed in lampreys. The edges of the funnel bear skin folds in lampreys, whereas in hagfish There are two pairs of mobile barbels that facilitate attachment. Deep within the funnel lies the rounded mouth opening, bounded ventrally by a powerful tongue armed with 1–2 sturdy horny teeth or a complex dental plate. Lampreys use their tongue to attach to the skin, while hagfish burrow into the victim's body. The Salivary Glands contain anticoagulants that prevent Blood clotting, as well as Proteolytic Enzymes.

The thin-walled Esophagus transitions seamlessly into the intestine (without loops) and opens via the anus. There is a large, compact Liver located posterior to The Heart. The Bile duct opens into the intestine. The Pancreas is scattered in islets along the intestinal wall and is invisible during routine dissection. Cyclostomes are capable of consuming large amounts of food. For instance, a hagfish consumes food weighing 7–8 times its own body weight within 7–8 hours. Cyclostomes can endure prolonged fasting lasting several weeks. Lampreys attach to fish—most commonly sick ones, occasionally very large ones (sturgeons, sharks, even whales). Small fish die from blood loss, while large ones often perish due to secondary infections entering through slow-healing wounds.

Hagfish sometimes attack in shoals. As many as 123 lampreys were once found inside a large cod, consuming its Internal Organs. Cyclostomes readily feed on carrion and small invertebrates. They search for prey using their SENSE OF SMELL. They hunt at night, whereas during the day they typically burrow into the silt, leaving only the anterior part of the head exposed.

Respiration and gas exchange. Lampreys have 7 pairs of gill pouches, each opening outward through an independent aperture and inward into the respiratory tube. Hagfish have 5–16 pairs of gill pouches that open outward via a common aperture. Respiration is driven by rhythmic contractions of the muscular wall of the branchial region. Water enters the Pharynx (or the respiratory tube in adults). In feeding cyclostomes, water enters and exits through the external openings of the gill pouches. Cutaneous respiration presumably also takes place.

The Circulatory system is structurally similar to that of amphioxus. However, cyclostomes develop a true heart consisting of an atrium and a ventricle. There is a single circulatory loop, and the heart carries exclusively venous blood. The atrium is adjoined by the sinus venosus, into which all venous vessels empty. Afferent branchial Arteries branch off from the ventral aorta to supply blood to the gill lamellae. Efferent branchial arteries empty into an unpaired aortic ROOT, from which carotid arteries extend anteriorly, while posteriorly the aortic root continues as the main arterial trunk — the dorsal aorta. It runs beneath the notochord. From the dorsal aorta, arterial blood is distributed via arteries to all organs. The Spleen is absent in cyclostomes. Hematopoiesis takes place in the walls of the esophagus and intestine, as well as in the Kidneys, liver, and adipose tissue.

Nervous System and Sense Organs. The Nervous System is more primitive than that of other vertebrates. The brain is relatively small; its divisions lie in a single plane and do not overlap one another.

The brain possesses all five typical vertebrate divisions: Forebrain, Diencephalon, mesencephalon (Midbrain), Cerebellum, and Medulla Oblongata. The forebrain is small; the olfactory lobes are noticeably larger than the forebrain and are weakly demarcated from it. The diencephalon features two optic nerves and a weakly developed Pituitary Gland (hypophysis) due to the simplicity of movements. The midbrain is represented by small optic lobes, reflecting a low level of Vision. Situated posterior to the midbrain is a very small cerebellum. The elongated medulla oblongata transitions imperceptibly into the ribbon-like spinal cord. Both the spinal cord and the Peripheral Nervous System exhibit a high degree of autonomy.

A decapitated lamprey or hagfish begins to perform swimming movements upon being stimulated. The sense organs correspond to the low level of ORGANIZATION OF THE nervous system. Olfactory organs are constructed very simply (a nostril, nasal passage, and olfactory capsule).

Lateral line organs (seismosensory organs) perceive water currents, detecting the approach of other animals or obstacles. Visual organs are paired eyes that possess a typical vertebrate Structure, but are covered by translucent skin. In hagfish, the eyes are less developed. Lampreys can distinguish only the outlines of underwater objects, and even then, only at a very close distance.

A weak electric organ is located on the head of sea lampreys and hagfish, generating pulses 50–80 milliseconds in duration and creating a low-voltage electric field. Nerve endings capable of perceiving Temperature, chemical, and mechanical stimuli are distributed throughout the skin.

Excretory organs are represented by mesonephric kidneys. Using a filtering apparatus, the kidneys have The ability to remove excess water and Metabolic waste products from the body. Alongside the skin, they participate in water-salt balance, maintaining the osmotic constancy of the internal environment. Like other vertebrates, cyclostomes first develop a pronephros (head Kidney). Later, paired trunk kidneys (mesonephros) develop posterior to it, which function in adult individuals.

Reproductive System and reproduction. All cyclostomes are dioecious (gonochoristic). In sexually mature individuals, the unpaired, multi-lobed gonad (Testis in males and Ovary in females) occupies almost the entire Abdominal cavity. Special ducts are absent; upon rupture of the gonad wall, mature Gametes spill into the body cavity, enter the urogenital sinus through genital pores, and are expelled outside via the urogenital opening. Fertilization is external. Brook lampreys, for example, spawn 1.5–3 thousand eggs, larger species such as the European and Caspian lampreys spawn 20–40 thousand, and large marine lampreys produce about 240 thousand.

Female hagfish spawn 15 to 30 oval-shaped eggs, 1–2.5 cm in length, at depths of 100 meters or more.

The development and growth of lampreys and hagfish differ significantly. In lampreys, a larva known as an ammocoete (about 1 cm long) hatches within 3–12 days and resembles amphioxus in its lifestyle. Metamorphosis takes 4–5 years to complete, after which the larva transforms into an adult lamprey. Hagfish development, on the other hand, occurs without metamorphosis. In anadromous forms, the juveniles migrate to the sea after metamorphosis, where they actively feed and grow, beginning their spawning migration several years later.

Behavior and lifestyle. Cyclostomes lie in wait for their prey, and upon detecting it using their Sensory Organs, they approach and attach to it. Prior to spawning, lampreys cease feeding and undertake spawning Migrations, sometimes traveling considerable distances upstream. Spawning marks the end of The life cycle for the majority of lamprey species. Hagfish inhabit coastal marine waters, moving to deeper offshore areas during the winter and for reproduction.

Ecological and economic significance. While cyclostomes play a relatively minor role in biocenoses, as semi-parasitic predators they impact the population numbers of their prey, occasionally causing noticeable damage to valuable commercial fish stocks. Certain lamprey species are of commercial importance; however, lampreys are scarce nearly everywhere and are harvested only in small quantities.

Today, there are approximately 40 extant species, which are classified into two subclasses: Hagfish (Myxini) and Lampreys (Petromyzones).