Salient Features and Classification of Animals

Introduction

Food is required for animals to grow and develop. Every animal is heterotrophic, meaning they eat both live and dead organic stuff. They are distinguished from autotrophic creatures which create their own nutrition through photosynthesis, and fungi, which digest their food externally, by this method of acquiring energy. Carnivores, herbivores, omnivores, and parasites are all examples of animals. The majority of animals reproduce in a sexual manner. Animal kingdom is mainly divided into two groups – vertebrates and invertebrates.

Classification of animals into groups is based on morphological and developmental traits such as body plan. The animal body plan is symmetrical (radial or bilateral symmetry), with the exception of sponges which are asymmetrical. This indicates that their body parts are distributed evenly along an axis. The levels of organisation, number of tissue layers generated during development, the presence or absence of an internal body cavity and other elements of embryological development are also factors in animal classification.

Phylum – Porifera

• Porifera includes organisms with holes

• They are multicellular primitive organisms with a cellular level of organisation

• They are non-moving creatures that are tethered to something substantial and usually asymmetrical with a canal system for transportation

• Water enters the body wall through ostia and flows into the spongocoel from which it exits through the osculum

• A skeleton made up of spicules or spongin fibres supports the body

• Internal fertilisation occurs, and development is indirect, with a larval stage that differs morphologically from the adult. Examples: Sycon, Spongilla and Euspongia

Phylum – Cnidaria or Coelenterata

• Cnidarians have stinging capsules or nematocysts on the tentacles and body, which are called cnidoblasts or cnidocytes employed for prey capture, defence, and anchoring

• Diploblastic, radially symmetrical aquatic, usually marine sessile or free-swimming organisms with tissue level organisation

• Polyp and medusa are the two primary body types of cnidarians. The former, like Hydra, is sessile and cylindrical, whereas the later, like jellyfish, is umbrella-shaped and free-swimming

• Polyps create medusa asexually, while medusa form the polyps sexually; those cnidarians that exist in both forms demonstrate alternation of generation (Metagenesis) (e.g., Obelia)

• Extracellular and intracellular digestion are both involved in digestion

Examples – Aurelia, Physalia, Adamsia, Pennatula, Gorgonia, and Meandrina

Phylum – Ctenophora

• Often known as sea walnuts or comb jellies

• They are diploblastic, marine, radially symmetrical and have tissue organisation

• Body consists of eight rows of ciliated comb plates on the outside which helps in movement

• Ctenophores exhibit bioluminescence

• They are hermaphrodite, and sexual reproduction is the only way to reproduce

• External fertilisation is combined with indirect development

• Pleurobrachia and Ctenoplana are two examples

Phylum – Platyhelminthes

• They are symmetrical, triploblastic and have organ level organisation

• The bodies of Flatworms are flattened dorsoventrally, or from top to bottom

• They might be parasitic or free-living. In parasitic forms, hooks and suckers are present

• They are acoelomate, hermaphrodite, fertilisation is internal and development occurs indirectly

• Flame cells are specialised cells that aid in osmoregulation and excretion

Phylum – Aschelminthes

• The body of an aschelminthes is cylindrical (bilaterally symmetrical)

• They are triploblastic, have a pseudocoelom and organ-system-level bodily organisation

• These parasitic worms are well-known as disease-causing parasitic worms, such as the worms that cause elephantiasis (filarial worms) or worms in the intestines (roundworm or pinworms)

• Roundworms get their name from the fact that their bodies are circular in cross-section

•  An excretory tube transfers body wastes from the body cavity

• Males and females are dioecious and females are frequently taller than males

• Internal fertilisation occurs, and development may be direct or indirect

Phylum – Arthropoda

• Arthropods, which include insects, arachnids, and crustaceans, are the world’s largest group of organisms

• Arthropods have segmented bodies and jointed legs, as well as hard, exterior shells termed “exoskeletons”

• Prawns, butterflies, spiders, scorpions are some well-known examples

• Triploblastic, segmented, have an organ system and coelomate. Blood fills the coelomic cavity

• Head, thorax and abdomen make up the entire body

• Circulatory system is open type. Respiratory organs are Gills, book gills, book lungs, and the tracheal system

• There are sensory organs like antennae, eyes, statocysts, and balancing organs

• Malpighian tubules are used for excretion

• They are dioecious, oviparous, fertilisation is internal and development may be direct or indirect

Phylum – Mollusca

• Mollusca is the second most populous animal phylum

• They are coelomate, bilaterally symmetrical, triploblastic and have an organ system

• Circulatory system is open type and excretory organs are like kidneys

• The radula is a file-like rasping organ present in the mouth which is used for feeding

• They are dioecious, oviparous, and their development is indirect

• The body is covered in a calcareous shell and is unsegmented, with a distinct head, muscular foot, and visceral hump. A soft and spongy layer of skin creates a mantle over the visceral hump

• Octopus, snails, and mussels are some examples

Phylum – Echinodermata

• Echinoderms have a calcareous ossicle endoskeleton

• They are free-living sea animals with organ-system organisation

• Triploblastic with a coelomic cavity. Echinoderm larvae are bilaterally symmetrical, whereas adult echinoderms are radially symmetrical

• For locomotion, food capture and transport, and respiration, a water-driven tube system [water vascular system] is used

• The digestive system is complete. Excretory system is absent

• The sexes are distinct. Fertilization is mainly done outside of the body

• Example – Starfish, Brittle star, sea urchin, sea lily, sea cucumber

Phylum – Hemichordata

• Hemichordata is classified as a distinct phylum within non-chordata

• It is made up of a tiny group of worm-like marine organisms that are organised at the organ-system level

• They are triploblastic, cylindrical [bilaterally symmetrical] coelomates

• The circulatory system of the body is of the open kind, gills are used for respiration, excretory organ is present

• The sexes are distinct. Fertilization takes place outside the body

• Balanoglossus and Saccoglossus are two examples

Phylum – Chordata

• They have a notochord, a dorsal hollow nerve cord, and paired pharyngeal gill slits

• They have a closed circulatory system and a post anal tail

• Urochordata or Tunicata, Cephalochordata, and Vertebrata are the three subphyla of the Chordata phylum

• The notochord is only present in the larval tail of Urochordata, but it extends from the head to the tail area of Cephalochordata and is present throughout their lives

• Ascidia, Salpa, and Doliolum are examples of Urochordata; Amphioxus or Lancelet are examples of Cephalochordata

Vertebrata

• They have a vertebral column and internal skeleton

• During the embryonic period, Vertebrates have a notochord

• In adulthood, the notochord is replaced by a cartilaginous or bony vertebral column

• Vertebrates have a ventral muscular heart with two, three, or four chambers, kidneys for excretion and osmoregulation, and paired appendages that can be fins or limbs, in addition to the basic chordate characteristics

• Vertebrates are symmetrical on both sides, triploblastic, coelomic, and segmented, having sophisticated body tissue and organ differentiation

Conclusion

Kingdom Animalia consists of organisms that range from the simplest of the animal forms to the most complex. At one end of this classification of animals, you have the microscopic organisms. Classification of animals helps us to understand their characteristics, as well as their differences with other organisms.