Tag: classifications of chordata

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.