Tag: classifications of chordata

Sea Lamprey or Petromyzon is an agnathan. It is a marine organism. It lacks jaws and resembles an eel. It has a sucker-like mouth that is involved in filter feeding. It has seven brachial openings behind its eyes. They are anadromous and are usually found at depths of around 4000 m and can tolerate temperature ranges from 1.0 and 20 °C.

Agnathans are basically primitive jawless fishes. They include lampreys and hagfishes. They usually have soft, cylindrical bodies that lack scales. In some species, separate external gill slits are present, while they open via a single pore in others. They have a suctorial or a filter-feeding mouth and they lack jaws.

So, the correct answer is 'Agnatha'.

The class Agnatha consists of a group of animals, that lack jaws and paired fins. There are two living groups of Agnatha, the lampreys, and the hagfish. 

Cyclostomata is a group of chordates comprising of living jawless fishes, the lampreys, and hagfishes. Their tail is protocercal in cyclostomes, having a caudal fin extending around the end of the vertebral column. The protocercal tail is formed by the division of tail into two equal lobes supported by fins.

Out of the given options, only Petromyzon is a jawless vertebrate, that is, it is an agnathan. In physical appearance, it resembles an eel. It has a sucker-like mouth that is involved in filter feeding. The rest of the examples are all gnathostomes or jawed vertebrates. 

Cyclostomata is a group of chordates that comprises the living jawless fishes, the lampreys, and hagfishes. They lay numerous eggs, about 1mm in diameter and lodge in the gravel around the nest. The egg hatches into an ammocoete larva, which is a blind, worm-like animal that burrows in the slit.

Petromyzon is the sea lamprey, which lacks bony teeth and jaws. Hagfish is eel-shaped, slime-producing marine fish. Along with lampreys, they are jawless and are living fossils.

Lamprey and hagfish or Petromyzon and Myxine are both agnathans as they are jawless fishes. They are sometimes considered as the earliest vertebrates. They also lack paired fins or stomach. They have a pineal eye. The adults and larvae usually have a notochord.

Vertebrates are the subphylum of a Chordata. Placoderms were among the first jawed fish. The jaws are considered to evolve from the gill arches. Placoderms are the extinct class of armoured prehistoric fishes, known from fossils, which lived from the Silurian to the end of the Devonian period. Their head and thorax were covered by articulated armoured plates and the rest of the body was scaled or naked.