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.

Cyclostomes are the organisms which belong to the group of chordates. These are the jawless fishes. The group also includes the lampreys and hagfishes. They have rounded mouths that lack jaws. The water keeps on cycling through the open mouth which cannot be closed due to lack of jaws. 

Carassius is a genus in the ray-finned fish family Cyprinidae.
Hippocampus is the name of marine fishes in the genus Hippocampus.
While Lepisma is a genus of primitive insects.
So, the correct answer is option D.

The hippocampus is the name of marine fishes.
Torpedo is the group of flattened cartilaginous fishes, also called an electric ray.
Mystus is a genus of small to medium-sized bagrid catfishes.
Petromyzon is the 'sea lamprey', which is parasitic lamprey. It is a 'jawless' fish.

In cyclostomes, the pineal gland is a photoreceptor. The endocrine nature of the gland's function has not been definitely established. As it's activity diminishes with age, the gland turns into rudimentary organ containing mineral deposits.

Cyclostomes are animals with a circular mouth, eel-shaped body with jaw less mouth, single median nostril and no paired fins. So, the correct answer is option B.

Myxine is a genus of hagfish, which are eel-shaped, slime-producing marine fish. 
Petromyzon is a parasitic lamprey found in the northern Atlantic ocean along shores of Antartica and North America, in the western Mediterranean sea and in the shores of the Great lakes. It is brown, gray, or black on its back and white or gray on the underside and can grow up to 90 cm long. 

Ammocoete larva is the larval stage of the primitive jawless fish, called as the lamprey or Petromyzon. Adult lampreys spawn in rivers and then die. The young larvae, ammocoetes, spend several years in the rivers, where they live burrowed in fine sediment, filter feeding on detritus and microorganisms. Then, ammocoetes undergo a metamorphosis lasting several months. 

• Agnatha is a superclass of jawless fish i.e these animals have no jaws.
• They come under the phylum chordata, subphylum vertebrata and includes the Cyclostomes and extinct Ostracoderm species.
• Placodermi is a class of armoured prehistoric fish having plated skin belonging to the superclass Gnathostomata (having jaws).
  
• Chondrichthyes is a class which contains cartilaginous fishes and belong to the superclass Gnathostomata.
  
• So the correct answer is 'Placodermi and Chondrichthyes'.

• Cyclostomata is a group of jawless living fishes which belong to the superclass Agnatha.
• Eg- the lampreys and hagfishes.
  
• The name Cyclostomata means circular mouths.
  

Cephalochordates are a group of organisms in which the notochord extends from head to tail region and is persistent throughout their life. Agnatha is a group of vertebrates that lacks jaw and notochord is present in both larvae and adult forms. While in Gnathostomata (jawed vertebrates) notochord is replaced by a vertebral column in adults.

• Phylum Chordata can be divided into two categories:-

Agnatha is a superclass of jawless fishes in the phylum Chordata. These are ectothermic, meaning they do not regulate their own body temperature. They show slow metabolism in cold water. They do not have a proper stomach rather they have a long homogenous gut. They are lowest grade vertebrates. The notochord is persistent in them. 

The sea lamprey (Petromyzon marinus) is a parasitic lamprey found in the northern Atlantic Ocean along shores of Europe and North America, in the western Mediterranean sea and in the shores of the Great Lakes. It belongs to superclass Agnatha (It is a superclass of jawless fishes). It is brown, gray, or black on its back and white or gray on the underside. The lamprey uses its suction cup-like mouth to attach itself to the skin of a fish and rasps away tissue with its sharp, probing tongue and keratinized teeth.

Petromyzen or Lamprey is a living fossil. The sea lamprey lacks bony teeth and jaws. It has an oral armament of conical teeth made of keratin. The lamprey even has teeth on its tongue. The oral apparatus of the adult lamprey is a sucking disk lined with whorls of over 100 of these teeth. 

Cyclostomata is a group of chordates that comprises the living jawless fishes - the lampreys and hagfishes. These fishes are a parasite as well as scavengers. The body is long, then tubular and tail is flat. They are also known as false fish.

Adult lamprey may be characterised by toothed, funnel like sucking mouth. The common name 'lamprey' is probably derived from latin, 'lampetro' which may mean stone licker.

Petromyzon (lampreys) are all carnivorous and are true predators. The method of attack is very peculiar. It attack fishes, turtles etc and it attach themselves by suctorial mouth which is circular in shape. Mouth acts a sucker, it rasps the flesh of the prey with its tongue. Teeth which are situated around the mouth sink into the flesh of the prey and serve it to hold properly. 

Metazoa is the term given to the fully developed complex organism having tissue level organization. Kingdom Animalia is considered as metazoa. All the members of this kingdom are multicellular, eukaryotes. They are heterotrophs, they depend on other organisms directly or indirectly for food. Most of the organisms are motile which means they can move independently and spontaneously. 

Vertebrata is the most important part of chordate and is distinguished by a backbone (made either of bone or cartilage) having bones known as vertebrae and a skull enclosing a brain. The nervous system is entirely protected by these structures and also maintains the shape of the body.

Vertebrata is the most important part of chordate and is distinguished by a backbone (made either of bone or cartilage) having bones known as vertebrae and a skull enclosing a brain. The nervous system is entirely protected by these structures and also maintains the shape of the body.

In bioluminescence, stored chemical energy on oxidation emits lights.

• They are jawless.
  
• They have a single median nostril.
• Their mouth is suctorial and circular.
• Body is elongated but no paired appendages or fins are present i.e. fins are unpaired.

1. All mammals are not Viviparous. Four species of Echidna and the Platypus are oviparous.
   2. All cyclostomes are jawless and lack paired fins.
   3. Crocodile is a reptile but has a four chambered heart unlike all other reptiles.
   4. Only the member of osteichthyes have gills covered by operculum while members of chondrichthyes lack operculum.
   So, the correct option is 'All cyclostomes do not possess; jaws and paired fins'.

• Gnathostomata consists of species of fishes which have jaws (jawed vertebrates).
  
• Myxine is a genus of hagfishes which are jawless (superclass- Agnatha).
• Loligo is a genus of squids belonging to the phylum Mollusca.
  
• Pristis is a genus of sawfish which is cartilaginous and belongs to the superclass Gnathostomata.
  
• Salamanders are animals having a slender body, blunt snout and short limbs which belong to the class- Amphibia.
  
• Calotes is a genus of lizards belonging to the class- Amphibia.
  
• Lancelets are fish-like marine chordate animals which are also known as amphioxi.
  
• So, the correct answer is 1.

Agnatha is a class of Chordata phylum. These are the jawless vertebrates. They have two nostrils but nares are absent. Chondrichthyes are the class of cartilaginous fishes. They are jawed vertebrates. In this, external nares are present whereas internal nares are absent. Osteichthyes are the class of bony fishes. They are jawed vertebrates. In this, external nares are present whereas internal nares are absent. 

Cyclostomata is characterized by round mouths. It is a group of chordates that comprises the living jawless fishes, examples are lampreys and hagfishes. They lack jaws which are replaced by retractable horny teeth. Their mouths cannot close due to the lack of a jaw, so they have to constantly cycle water through the mouth.

The jawed animals have three semicircular canals which are mainly statoreceptors which help to maintain the balance. Hagfish has a single semicircular canal which contains both stereocilia and the second class of hair cells, whereas other vertebrates have stereocilia only. Because the inner ear of hagfish has two forms of sensory ampullae, their single semicircular canal is assumed to be a result of two semicircular canals, that have merged into just one.

 Petromyzon, sea lamprey has a simple nasal aperture is present on the top of the head between the eyes. Nasal aperture is single and median. Hence, option A is correct answer.

The class Cyclostomata is characterized by circular mouth without jaws. They have retractable horny teeth. Also, the scales and fins are absent in these members of the class. They are ectoparasites and don't have mucus glands on their skin.

Ostracoderms refers to the armored jawless fishes of the Paleozoic era. They lived in what is now North America, Europe, and Russia from the Middle Silurian to Upper Devonian. They were the first fish to use gills just for respiration. This is the first class of vertebrates. 

Agnatha which means 'no jaws' is a superclass of jawless fish in the phylum Chordata, consisting of both present (cyclostomes) and extinct species like ostracoderms. The body is covered in skin, with neither dermal or epidermal scales. Hagfish and Lampreys are a very common example. 

Petromyzon (lampreys) are all carnivorous and are true predators. The method of attack is very peculiar. It attack fishes, turtles etc and it attach themselves by suctorial mouth which is circular in shape. Mouth acts a sucker, it rasps the flesh of the prey with its tongue. Teeth which are situated around the mouth sink into the flesh of the prey and serve it to hold properly. 

Cyclostomata is a group of chordates that comprises the living jawless fishes: lampreys and hagfishes. Both groups have round mouths that lack jaws but have retractable horny teeth. The name Cyclostomata means "round mouths". These fishes are parasites and as well as scavengers. They feed off dead and wounded organisms in the ocean. 

The subphylum vertebrae are  classified into Agnatha or Agnathostomata- includes vertebrates without jaws. Further, this group has only one class the Cyclostomata. The Cyclostomes are jawless vertebrates with a circular mouth and suctorial funnel.

Petromyzon (lampreys) are all carnivorous and are true predators. The method of attack is very peculiar. It attack fishes, turtles etc and it attach themselves by suctorial mouth which is circular in shape. Mouth acts a sucker, it rasps the flesh of the prey with its tongue. Teeth which are situated around the mouth sink into the flesh of the prey and serve it to hold properly. Hence, option D is correct answer.

Agnatha is a superclass of jawless fishes in the phylum Chordata. These are ectothermic, meaning they do not regulate their own body temperature. They show slow metabolism in cold water. They do not have a proper stomach rather they have a long homogenous gut.
So, the correct answer is option B.

Myxine is scientific name of hagfish. They are eel-shaped, slime-producing marine fish. They are the only known living animals that have a skull but no vertebral column. Classification of Myxine is

Humans are categorized as vertebrates in the phylum Chordata. The phylum Chordata contains all animals that possess, at some point during their lives, a hollow nerve cord, and a notochord, a flexible rod between the nerve cord and the digestive track. The phylum Chordata is an extremely diverse phylum and the one most recognizable to us.