Phylum chordata

The phylum being described is Phylum Chordata, a significant and diverse group of animals that includes humans and all other known animals and birds. One distinguishing feature shared by all chordates is the presence of a notochord, alongside other characteristic traits such as a dorsal nerve cord, post-anal tail, and pharyngeal slits, which appear at various stages in their life cycles.

Phylum Chordata is categorized into three subphyla:

Urochordata: This subphylum encompasses organisms like tunicates or sea squirts, which are marine filter feeders with relatively simple body structures.

Cephalochordata: Cephalochordates, such as lancelets, are small, marine organisms that maintain chordate characteristics throughout their entire life cycle.

Vertebrata: The largest and most diverse subphylum, Vertebrata, comprises animals with a vertebral column made of bone or cartilage. This subphylum includes various classes of vertebrates, including fish (Pisces), amphibians (Amphibia), reptiles (Reptilia), birds (Aves), and mammals (Mammalia). Vertebrates are known for their well-developed nervous systems, circulatory systems, and skeletal structures that provide support and protection.

Class – Cyclostomata

The members of the Cyclostomata class are notable for their unique characteristics and parasitic lifestyle. These organisms are ectoparasites that attach themselves to specific fish species. They exhibit the following distinctive traits:

Body Shape: Cyclostomata organisms have elongated bodies.

Respiration: They possess 6-15 pairs of gill slits, which they use for breathing.

Mouth Structure: Unlike most other creatures, Cyclostomata have a specialized circular mouth without jaws.

Body Features: These organisms lack scales and paired fins.

Skeletal Structure: Their cranium (skull) and vertebral column (backbone) are made of cartilage rather than bone.

Circulatory System: Cyclostomata have a closed circulatory system.

Habitat and Reproduction: While they primarily inhabit marine environments, Cyclostomata migrate to freshwater for spawning. Following reproduction, they typically have a short lifespan, with their larvae undergoing metamorphosis and returning to the ocean.

Class – Chondrichthyes

These marine animals are characterized by their streamlined bodies and possess a cartilaginous endoskeleton. Their mouths are situated on the ventral side, and they maintain a persistent notochord throughout their entire lives. These creatures have separate gill slits without an operculum (gill cover), and their skin is tough, featuring minute placoid scales. Their teeth are modified placoid scales that are backwardly directed, and they have powerful jaws, making them predatory animals.

To stay afloat, they must continuously swim as they lack an air bladder. Their hearts consist of two chambers, one auricle and one ventricle. Some species, like the Torpedo, possess electric organs, while others, like the Trygon, have poison stings. They are cold-blooded, meaning they are unable to regulate their body temperature (poikilothermic). These animals have separate sexes, with males having claspers on their pelvic fins. They reproduce through internal fertilization, and many of them are viviparous, giving birth to live young.

Notable examples of these creatures include Scoliodon (Dogfish), Pristis (Sawfish), Carcharodon (Great White Shark), and Trygon (Stingray).

Class – Osteichthyes

This group encompasses a wide variety of fishes found in both marine and freshwater environments. They are characterized by their bony endoskeleton and have streamlined bodies. Typically, their mouths are located at the terminal end of their bodies.

These fishes possess four pairs of gills, each protected by an operculum on both sides. Their skin is adorned with cycloid or ctenoid scales, and they are equipped with an air bladder that helps them control their buoyancy in the water. Their hearts consist of two chambers, one auricle and one ventricle, and like most fish, they are cold-blooded, unable to regulate their body temperature (poikilothermic).

In terms of reproduction, they have separate sexes, and fertilization typically occurs externally. The majority of these fishes are oviparous, laying eggs with direct development.

Notable examples include Exocoetus (Flying Fish) and Hippocampus (Seahorse) in marine environments, while freshwater species include Labeo (Rohu), Catla (Katla), and Clarias (Magur). Additionally, some species like Betta (Fighting Fish) and Pterophyllum (Angelfish) are popular choices for aquarium enthusiasts.