Classification of Kingdom Animalia 3

 

8- Phylum Echinodermata

They are exclusively marine animals. Some are flattened like biscuit (e.g., cake urchin), some are star-shaped with short arms (e.g., sea star or starfish), some are globular (e.g., sea urchin), some are star-shaped with long arms (e.g., brittle star), and some are elongated (e.g., sea cucumber).

They are triploblastic and deuterostomes coelomates. Their larvae are bilateral symmetrical but the adults show radial symmetry. In their radial symmetry, the body parts are arranged in five, or multiple of five, around an oral-aboral axis. They possess a calcareous endoskeleton in the form of plates called ossicles. The ossicles are derived from mesoderm but come out of skin also and make spines on it. 

They possess a water-vascular system consisting of tubes and spaces present in the coelom. A ring canal surrounds the mouth. It opens outside through a sieve-like plate, called madreporite. Five (or a multiple of five) radial canals branch from the ring canal. Many lateral canals emerge from each radial canal and each lateral canal ends at a tube foot. Tube feet are the extensions of water vascular system. The tube feet extend and attach with some substrate. When water is drawn back from the sucked tube feet, they contracts and body is pulled. Echinoderms possess tube-like digestive system. The mouth leads to oesophagus, stomach, intestine and rectum. The rectum opens out through anus.

There are no specialized organs for respiration and excretion. They possess a poorly developed nervous system made of a nerve net, a nerve ring, and five (or multiple of five) radial nerves. Most sensory receptors are distributed over the surface of the body and tube feet. Asexual reproduction involves division of the body, followed by the regeneration of each half. Echinoderms are unisexual.

Many echinoderms are able to regenerate the lost parts, and some, especially sea stars and brittle stars, drop various parts when they are under attack and then regenerate the lost parts.

9- Phylum Chordata

Chordates are bilateral symmetrical, triploblastic, deuterostome coelomates. The following four characteristics are unique to chordates, present at some stage in development.

i)Notochord: All chordates develop notochord during embryonic life. It is a rod-like semi rigid body of vacuolated cells. It extends throughout the length of body between gut and dorsal nerve cord. The lower chordates retain this notochord throughout life. While, in vertebrates it is partly or entirely replaced by vertebral column, during development.

ii)Pharyngeal slits: These are a series of openings in the lateral walls of pharynx. All chordates develop paired gill slits in embryonic stage. In some chordates (e.g.,Amphioxus and fishes), these develop into gills. In some (e.g., most amphibians), these are functional for some period in their life history. In others (e.g., reptiles, birds and mammals), these are modified for various purposes.

iii)Tubular nerve cord: In all chordates, a tubular nerve cord runs through the longitudinal axis of the body, just dorsal to the notochord. It expands anteriorly as a brain.

iv)Post anal tail: All chordates develop a tail, posteriorly beyond the anal opening. Some chordates retain it throughout life while others degenerate it during embryonic life.

Phylum chordata includes two major groups i.e., invertebrate chordates and vertebrates.

Subphylum Urochordata includes the invertebrates chordates in which notochord and nerve cord are present only in their free-swimming larvae. Sea squirts are the examples of urochordates.

Subphylum Cephalochordata includes the invertebrate chordates in which notochord persists throughout life. Amphioxus is a common cephalochordate.

Vertebrates: They have a vertebral column and cranium. Vertebrates are divided into seven classes which are placed into two groups.

CLASSIFICATION OF VERTEBRATES

Vertebrates are divided into two groups.

Group Pisces: It includes 3 classes i.e., Cyclostomata, Chondrichthyes, and Osteichthyes. They do not have limbs.

Group Tetrapoda: It includes 4 classes i.e., Amphibia, Reptilia, Aves, and Mammalia.

They have four limbs:

Class Cyclostomata

These are jawless fishes. Lampreys and hagfish are common examples. Their bodies are eel-like and not covered with scales. They possess cartilaginous skeleton. Like other fishes, they have a single-circuit heart with one atrium and one ventricle. Fertilization is external.

Class Chondrichthyes

The group includes sharks, skates, rays, and ratfishes. They have skeleton of cartilage. Their body is covered by placoid (tooth-like) scales, called denticles. They have jaws and biting mouthparts. The pectoral and pelvic fins are paired. There are two dorsal fins. They possess single-circuit heart with one atrium and one ventricle. There is a pair of small openings, called spiracle, behind eyes. These are used for breathing. They do not have swim bladder. Fertilization is internal.

Class Osteichthyes

The have bony endoskeleton, streamlined body, dermal bony scales, and terminal mouth with jaws (with or without teeth). Notochord is replaced by vertebral column, but some bony fishes may retain it in reduced form. They also have a swim bladder that helps in buoyancy. They possess both median (dorsal, caudal and ventral) and paired (pelvic and pectoral) fins. They contain four pairs of gills. A protective bony flap, operculum, protects the gills. They have well developed nervous system in which there are ten pairs of cranial nerves. 

Fertilization is mostly external. The freshwater bony fishes include rohu, trout, Katla, catfish etc. The marine bony fishes include seahorse, flying fish and angler fish etc.