Tag: characteristics of vertebrates

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.