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. 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

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 (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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