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.
Characteristics of Petromyzontiformes
Characteristics of myxiniformes
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.
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.
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.
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.
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.
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 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.
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.