4. Kingdom Animalia - part 13 - Subphylum : Vertebrata and its divisions,supaerclass Pisces and its two classes

 

4. Kingdom Animalia - part 13 - Subphylum : Vertebrata and its divisions,supaerclass Pisces and its two classes


Subphylum : Vertebrata
  • In these chordates, notochord is replaced by cartilaginous or bony vertebral column. 
  • It is divided into two divisions - 
  1. Agnathostomata (no jaws) and 
  2. Gnathostomata (jaws present).
1. Division : Agnathostomata :
  • This division includes the lowest or most primitive vertebrates, which are without jaws. 
  • They include only one class of living vertebrates - the Cyclostomata.

Class : Cyclostomata (Cyclos : Circular, Stoma - mouth) Lat/Grk.

  • e.g. Petromyzon (Lamprey), Myxine (Hagfish).
  • Cyclostomes are jawless and eel-like animals. 
  • Skin is soft, smooth containing unicellular mucus glands, but no scales. 
  • Median fins are present but paired fins are absent.
  • They are ectoparasites. 
  • They have sucking and circular mouth without jaws. 
  • Cranium and vertebral column made up of cartilage.
  • Digestive system lacks stomach.
  • Respiration occurs by 6 to 15 pairs of gills slits.
  • Gills slits are without operculum. 
  • Heart is two chambered with one auricle and one ventricle.
  • Gonad is single, large and without gonoduct. 
  • Fertilization is external. 
  • They are anadromous i.e. migrate for spawning to fresh water from their marine habitat. 
  • After spawning, they die within few days. 
  • Larvae metamorphose and then migrate to ocean.
2. Division : Gnathostomata
  • It is divided into two superclasses -
  1. Pisces (bear fins) and 
  2. tetrapoda (bear four limbs).
A. Superclass : Pisces
  • These are aquatic animals. 
  • These are poikilothermic (body temperature changes according to the change in surrounding temperature). 
  • Lateral line system is present which shows presence of rheoreceptores for detection of water current. 
  • Locomotion is by body muscles and fins. 
  • Caudal fin acts as steering wheel. 
  • Exoskeleton is of dermal scales.
  • Endoskeleton is either bony or cartilagenous.
  • Body is streamlined and boat shaped. This feature offers minimum resistance during swimming. 
  • Respiration is by gills. 
  • Heart is two chambered and is ventral in position. It shows single and closed circulation. 
  • Heart always shows presence of deoxygenated blood, so it is described as venous heart. 
  • They have well developed brain with large olfactory lobes.
  • Sexes are separate. 
  • Most fishes are oviparous and some are viviparous.
  • Superclass Pisces is divided into two classes as below.
  1. Class Chondrichthyes  
  2. Class : Osteichthyes


1. Class Chondrichthyes (chondron : cartilage, ichthyes : fish)  :


  • e.g. Scoliodon (dog fish), Pristis (sawfish), Electric ray, Common skate, Hammer headed shark.
  • Chondrichthyes includes the animals in which endoskeleton is cartilagenous. 
  • These are exclusively marine. 
  • Exoskeleton is formed of placoid scales. 
  • Teeth are modified placoid scales which are backwardly directed. 
  • Mouth is ventral in position. 
  • There is single dorsal fin and 2 pairs of lateral fins (pectoral and Pelvic).Caudal fin is heterocercal (Asymmetrical). 
  • Five to seven pairs of gill slits are present. They are not covered by operculum. 
  • Air bladder is absent hence these fishes need to swim constantly so that they do not sink. 
  • They are predatory fishes.
  • Some of them have electric organs e.g. Torpedo - (electric ray) and some have poison sting e.g. Trygon - (sting ray) as organs of offence ans defence. 
  • Male copulatory organs called claspers are present. 
  • Fertilization is internal. 
  • Many of them are viviparous.


2. Class : Osteichthyes (Osteon : bone , ichthyes : fish) :


  • e.g. Bombay duck, Lung fishes (Protopterus, Lepidosiren) Exocoetus (flying fish), H ippocampus (sea-horse), Pomphret, Labeo rohita (Rohu), Catla (Katla), Clarias (Magur), Aquarium fishes. Betta -(fighting fish), Pterophyllum (Angle fish).
  • Osteichthyes includes fishes in which bony endoskeleton is present, hence called as bony fishes. 
  • These are aquatic, present in both fresh and marine waters. 
  • Exoskeleton is formed of cycloid and ctenoid scales. 
  • Mouth is mostly terminal in position. 
  • They show two dorsal fins. Tail fin is formed by two equal lobes i.e. homoceral (symmetrical). 
  • Four pairs of gill slits are present, covered with operculum.
  • Air bladder is present to maintain buoyancy.
  • Claspers are absent. 
  • Fertilization is external.
  • These fishes are oviparous.


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