Characteristics of Invertebrates

The main characteristics of invertebrates are stated below.

• Habitat: Invertebrates are widely distributed on lands like Arthropods, freshwater, and sea such as sponges and coelenterates. About 80% of the invertebrates are terrestrial. Protozoans have free-living or parasitic life and sometimes live as commensals.
• Numbers: About 95% of all the animals in the animal kingdom are invertebrates.
• Body shape: Body shape varies among invertebrates. Amoeba has an irregular shape and flatworms have leaf-like or ribbon-shaped bodies whereas Starfishes have star-shaped bodies.
• Size: Size varies from microscopic to macroscopic. For example, Plasmodium are the smallest in size whereas giant squids are the largest one.
• Body Symmetry: The body can be asymmetrical, bilateral, or radially symmetrical. For example, Protozoans have bilateral, radial, or asymmetrical bodies whereas sponges have asymmetrical or radially symmetrical bodies.
• Tissue Grade of Organisation: They have all grades of organization like protoplasmic grade (Protozoa), cellular grade (Sponges), cell-tissue grade (Coelenterates), and tissue-organ grade (Flatworms).
• Germ Layers: Germ layers are diploblastic (Sponges and Coelenterates) or triploblastic (other than Sponges and Coelenterates). Germ layer absent in Protozoans.
• Simple Integument: Bodies have a simple covering known as the epidermis (except Protozoa). Protozoans have plasma membranes.
• Locomotion: Locomotory organs are cilia, flagella pseudopodia (Protozoans), setae, parapodia (Annelids), jointed legs (Arthropods), and arms (Echinoderms). Sponges and corals are sessile.
• Body Segmentation: Some have true segmentation (Annelids and Arthropods) and some have pseudo-segmentation (Flatworms).
• Living Endoskeleton: They do not have a rigid internal skeleton. But some possess hard exoskeletons (Arthropods and Molluscs).
• Coelom : Pseudocoelom or true coelom are present. Acoelomates have no coelom.
• Alimentary canal: It is either absent or present partially formed and in some, a complete alimentary canal is present dorsal to the nerve chord running from the anterior terminal mouth to the posterior terminal anus.
• Digestive System: Digestion is intracellular or extracellular or both.
• Circulatory System: The circulatory system is open or closed or lacunar. The heart is present in the dorsal side of the gut.
• Respiratory System: Respiration occurs through direct gas diffusion or skin or gills or tube feet or tracheal system.
• Excretory Mechanisms: Excretory organs are flame cells, true nephridia, malpighian tubules, or amoeboid cells.
• Nervous System: The central nervous system is represented by a ring of nerve tissue encircling the body or a pair of nerve cords running along the mid-ventral line of the body. In higher vertebrates, the brain is present in the form of head ganglia.
• Sense Organs: Sense organs include stigma, eyespots, chemoreceptors, simple eyes, compound eyes, statocysts, and tactile receptors.
• Reproduction: Reproduction is either asexual (binary fission) or sexual. Fertilization may be internal or external. Development is direct or indirect. Parthenogenesis occurs in some invertebrates like Rotifers and bees.
• Body temperature: All invertebrates are cold-blooded animals i.e. they can regulate body temperature.

Also Read: Difference Between Chordates and Non Chordates

The invertebrates are characterized by the absence of a vertebral column and they can be classified mainly into nine types according to the Phyla. Invertebrates are the largest group in the animal kingdom which are mainly included in the Phyla Protozoa, Porifera, Cnidaria, Annelida, Platyhelminthes, Aschhelminthes, Arthropoda, Mollusca, and Echinodermata.

Invertebrate animals play an important role as agricultural pests, parasites, or vectors of parasitic infections to humans and other vertebrates and also help in plant pollination. In this article, we will learn about different types of invertebrates along with their characteristics.