Introduction and classification of Agnatha with salient feature of Petromyzon

Agnatha

Agnatha are the primitive group of organism similar to fish. The scientist had recorded the fossils of agnathan species from the late Cambrian period. These are jawless organism in the phylum chordata and sub-phylum vertebrata. The Lampreys and hagfish are the two living groups of superclass Agnatha.

General characters

• Absence of jaw and pair of appendages.
• Body is cylindrical, elongated and eel like.
• Mouth is directly opened into intestine.
• Intestine is in folded structure called typhlosole.
• Presence of 6-15 pairs of gill, Gill pouch and gill slit are used in respiration.
• Unpaired fins are present which is supported by cartilaginous fin ray.
• Heart is two chambered i.e. auricle and ventricle.
• Lack of renal portal system but presence of hepatic portal system.
• Sex are united.
• These are cold blooded animal.
• External fertilization is occurred.
• Development is direct/ indirect.
• Mesonephric and nephridia are excretory organ.

Classification

Kingdom: Animalia

Phylum: Chordata

Sub-phylum: vertebrata

Superclass: Cyclostomata

Classes: Agnatha

Order

• Petromyzontiformes(petromyzon)
• Myxiniformes(maxine)

Characteristics of Petromyzontiformes

• These are jawless fish commonly called as lampreys.
• Found in the temperate river, estuaries, coastal seas and some species are in fresh water.
• Mouth is circular and sectorial without jaw and filled with row of horny teeth.
• Mouth is located in the ventral side of the body.
• Single nostril and nasal sac on the dorsal side of the body.
• Branchial baskets are well developed.
• Eggs are numerous and small size.
• Development is indirect.
• Fertilization is external
• Example; petromyzon

Characteristics of myxiniformes

• Commonly called as Hagfishes.
• Primitive eel like organism without eyes.
• Mouth is terminal with triangular teeth.
• Commonly found in the marine water.
• Four pairs of tentacles are present. Tentacles surround the mouth.
• Development is direct.
• Dorsal fin is weakly developed.
• Example; myxine

Petromyzon

Petromyzon are commonly called as lamprey having rounded body with compressed tail. Muscular tongue is present in the buccal cavity bearing the epidermal teeth and jaws are absent. The skin of petromyzon is soft and devoid of scales, paired appendages but median fins are present which is supported by cartilaginous fin rays. They are free living and most are parasite on fishes in the adult state. Scientist believed that these organisms are evolved in the Devonian.

Systematic position

Phylum:           chordata

Subphylum:     vertebrata

Division:          agnatha

Order:              petromyzontiformes

Family:            petromyzonidae

Genus:             Petromyzon

Species:           marinus

Distribution

They are cosmopolitan in distribution present in both fresh water as well as marine water. They are inhabitants of North America, Europe, West Africa, Japan, Chile, Australia, New Zealand etc.

• Habit and habitat

Lampreys are carnivorous animal prey upon fishes and turtle. In the larval stage they are free living, sedentary and filter feeding while in the adult stage they are ectoparasite. It has circular mouth which helps to attach on the body of host.

They usually live in the sea water attached to the body of fishes or turtles and migrate to the river during autumn. The purpose of migration is spawning. They get the sexual maturity in the winter season while breeding takes place in spring. After spawning lampreys usually die.

• Shape and size

The Body of Petromyzon is elongated, cylindrical and eel like. It has different species with different body size and length. The salty lampreys have a maximum 1m length while fresh water lampreys attain upto 90cm. for e.g. Petromyzon marinus has a meter length, Lampetra fluviatilis has about 90cm and Lampetra planeri has less than 45cm.

• Color of Body

Body color of small specimen’s usually blackish blue or as lead colored and silvery colored. But in large specimens having olive brown, greenish brown, green, red, blue mottled with darker shade of the color. The lower surface of organism has whitish or gray color.

• Division of Body

The elongated and cylindrical body of petromyzon is divided into three region i.e. head, trunk and tail which are not distinctly demarcated. Head is the anterior part of the body bearing a mouth, single nostril, pair of eyes and 7 pairs of gill pores. Head and trunk is elongated and cylindrical but the tail is less or more laterally flattened. Mouth is situated on the ventral position of the body which is opened into the funnel like structure called the buccal funnel. Single nostril is located mid-dorsally over the head between a pair of eyes. And 7 pairs of external gill slits are behind the eyes.

Median dorsal fin is divided into two unequal parts by notch situated near the posterior end as the junction between trunk and tail and a caudal fin is present around the tail. Anus is present on the ventral side of the body and on the top of ventral side of anus there is presence of urinogenital papillae

Petromyzon shows the sexual dimorphism. In the period of breeding season anal fin is developed and eversible penis likes structure in male.

Buccal funnel

Fig: ventral view of head showing details of buccal funnel.

Buccal funnel is special cup like structure developed in the head region which is downwardly directed depression. The buccal funnel is lined with papillae internally called as oral fimbriae. Between the papilla the long sensory cirri is present. The inside of buccal funnel is beset with radiating rows of conical yellow, horny, epidermal teeth. These are mesodermal in origin. The large tooth plate are formed by fusing the upper and lower side of tooth is called supra oral and infra oral tooth plates respectively. Mouth is circular opening situated in the apex of buccal funnel and which is surrounded by the concentric rows of lateral teeth each having two cusps thus called as bicuspid.             

Economic importance

• The flesh of lampreys is used as food in Europe and America.
• Lampreys are also used as bait.
• They destroy valuable fish by feeding on their blood and body fluid.

Reference

Kotpal, R.L. Modern Textbook of Zoology, vertebrates. New Delhi: Rostogi Publication, 2012.

Jordan, E.L. and P.S. Verma. Chordate Zoology. New Delhi: S. Chand & Company Pvt. Ltd, 2014.