The following points highlight the twenty-one general characteristics of invertebrates. Some of the characteristics are: 1. Habitat 2. Numerical Strength 3. Shape 4. Size 5. Symmetry 6. Grades of Organization 7. The Presence or Absence of Germ Layers 8. Simple Integument 9. Multiple Movement Device 10. The Presence or Absence of Segmentation and Others.
Characteristic # 1. Habitat:
All the 30 phyla most probably originated in the sea, but not all have successfully invaded the land or its freshwater habitats. About 80 per cent are found in the terrestrial habitats. No doubt Porifera, Coelenterata and Entoprocta are represented in freshwaters but by fewer species than in the seawaters.
Rotifera and Insecta are more, both in number of species and individuals, in freshwaters. Gastrotricha have about equal number of species in both fresh and seawaters.
Echinodermata and Brachiopoda have never successfully colonised freshwaters. It is true that earthworm is a terrestrial animal, but most annelids are marine. There are few land snails, but most molluscs are marine. Amongst arthropods, only Crustacea is largely marine, but spiders, centipedes, millipedes and insects are mostly terrestrial. Thus, the most successful invertebrates of the land comprise arthropods.
Characteristic # 2. Numerical Strength:
At present nearly one million living species of animals are known, out of which about 95 per cent constitute the invertebrates. It has been estimated that the number of extinct species is around seven times the number of living species and, therefore, there may have been some seven million species.
Characteristic # 3. Shape:
Animals of varied shapes are included amongst the invertebrates. Amoeba possesses an irregular ever-changing body shape, sponges and coelenterates display plant-like appearance, flatworms are leaf-like and ribbon-shaped and annelids, nemertean’s and nematodes are vermiform, while the starfishes are star-shaped, etc., display spectra of body shape.
Characteristic # 4. Size:
The invertebrate animals exhibit a great variation in size. They range from microscopic protozoans to large-sized cephalopods. The malaria parasite is at the lowest extremity. It occupies only about one-fifth of a human red blood corpuscle (RBC).
The uppermost extremity is occupied by a species of the giant squids of North Atlantic, Architeuthis, has been reported to have attained a total body length of 16.5 metres including tentacles.
Characteristic # 5. Symmetry:
Invertebrates represent all types of symmetries. Protozoans display bilateral as well as radial symmetry. Some are asymmetrical. Sponges are either asymmetrical or radially symmetrical. Coelenterates are radially symmetrical. Ctenophores exhibit biradial symmetry.
The members of the remaining phyla are mostly bilaterally symmetrical. Invertebrates also represent spherical symmetry, principally in spherical protozoans such as Heliozoa and Radiolaria.
Characteristic # 6. Grades of Organization:
Invertebrates display all grades of organisation. The protoplasmic grade is seen in Protozoa, as all activities at this level are carried on within the limits of plasma membrane (plasma lemma).
The cellular grade is characteristic of sponges. In sponges only, the cells exhibit division of labour for performing various specialised functions. The cell-tissue grade is observed in coelenterates as their cells are not only specialised for different functions but also certain similar cells gather together to form tissues as well.
A notable example is the nerve net formed by nerve cells and their processes. The tissue-organ grade is exhibited by flatworms with arrangement of tissues to form organs. The organ-system grade organisation is characteristic of all higher invertebrates. In this type of organisation, organs join together in a system to perform some functions.
Characteristic # 7. The Presence or Absence of Germ Layers:
The germ layers or embryonic cell layers are absent in Protozoa due to its unicellularity. All other invertebrates are either diploblastic, i.e., they are derived form two germ layers, an outer ectoderm and an inner endoderm or triploblastic with an extra third layer, the mesoderm. Sponges and coelenterates are diploblastic, whereas other invertebrates are triploblastic.
Characteristic # 8. Simple Integument:
The body covering of invertebrate animals is simple. In protozoa, it is a delicate plasma membrane, while some have Invertebrates represent all types, developed a protective covering, pellicle. Most invertebrates possess an outer protective epidermis, which is made of single layer of cells, while in others have further added a non-cellular cuticle or chitinoid covering secreted by underlying epidermis.
Characteristic # 9. Multiple Movement Devices:
Various devices for movement are found in invertebrates. Some invertebrate animals are sessile, such as sponges and corals, while others move from one place to another. Protozoa move by pseudopodia, flagella and cilia and contractile myonemes. Coelenterates and molluscs exhibit tentacular movements.
Annelids move by setae, parapodia and suckers. Arthropods move with jointed legs, while echinoderms take the help of arms which are with or without tube feet, for their movement.
Characteristic # 10. The Presence or Absence of Segmentation:
The members of several invertebrate phyla are characterised by segmentation in their bodies. Certain flatworms exhibit pseudo segmentation, as their long bodies are made up of numerous sections. True segmentation is found in Annelida and Arthropoda. In them the body is divided into more or less similar segments.
Characteristic # 11. Absence of Living Endoskeleton:
The invertebrate animals do not possess any kind of rigid internal skeleton to give support to the body and provide surface for attachment of muscles. Many invertebrates are soft bodied, while some, like arthropods and molluscs, possess hard exoskeleton for supporting and protecting their body.
Characteristic # 12. Types of Coelom:
In sponges and coelenterates, the body is a double-layered sac surrounding a single cavity, which opens to the outside through a mouth. Such animals are acoelomate as they have no coelom. Other invertebrates possess a cavity in between the body wall and the gut.
This cavity is called pseudo-coelom in nematodes as it is not lined by mesoderm. In higher invertebrates, the coelom is lined by mesoderm and, hence, it is the true coelom.
Characteristic # 13. Dorsal Gut:
The alimentary canal is either absent or partially formed or complete. In case, it is present, it lies dorsal to the nerve cord, and runs from the anterior terminal mouth up to the posterior terminal anus. The gill-slits are never formed in the pharyngeal wall.
Characteristic # 14. Intra-as well as Extracellular Digestion:
In invertebrates, the digestion of food takes place within the cell (intracellular digestion) as well as outside the cell (extracellular digestion). In protozoans and sponges, the digestion of food takes place intracellularly.
In coelenterates, the digestion of food lakes place both intracellularly as well as extracellularly. All other invertebrates exhibit extracellular digestion, which in higher invertebrates occurs within a well defined gut.
Characteristic # 15. Open as well as Closed Circulatory System:
Blood vascular system is well developed in higher invertebrates. Some, like arthropods and molluscs, possess open or lacunar circulatory system, while in others the blood flows in closed vessels, i.e., closed circulatory system. The heart is always located dorsal to the gut. The hepatic portal system, carrying blood from the gut to the liver, is absent.
Characteristic # 16. Diversified Respiratory System:
Protozoans, sponges, coelenterates and many worms have a direct diffusion of gases between the general surface of the organism and the environment. In most annelids, the exchange of gases takes place through the moist skin. Gills are common in most higher invertebrates.
Echinoderms possess branchiate and tube feet for this purpose. Sea cucumbers have respiratory trees which act as respiratory organs. In insects, the tracheal system is adapted for aerial respiration.
Characteristic # 17. Diversified Excretory Mechanisms:
In protozoans, sponges and coelenterates, excretion is performed by direct diffusion through cell membranes. Flatworms possess characteristic flame cells, while annelids and molluscs possess true nephridia for the purpose. In insects, the excretory organs are Malpighian tubules. Echinoderms and some other invertebrates have amoeboid cells or phagocytes for storage and disposal of excretory products to outside.
Characteristic # 18. Ventral Solid Nervous System:
In radially symmetrical invertebrates, e.g., coelenterates, the head is absent and the central nervous system is represented by a ring of nerve-tissue encircling the body. In bilaterally symmetrical invertebrates, the central nervous system comprises a pair of nerve cord running along the mid-ventral line of the body.
The nerve ring and the nerve cords bear ganglia. In higher invertebrates, the head ganglia forms the brain. Invertebrate nervous system is characterised by solid nerves, these are not hollow within.
Characteristic # 19. Simple Sense Organs:
In protozoans, the whole of the protoplasm acts as receptor, while in flagellates, the stigma or eyespot acts as a photoreceptor. Coelenterates possess long sensory cells, scattered throughout the body wall, while some also possess eyespots for the reception of light, statocysts for equilibrium and sensory pits for chemoreception.
Eyespots and chemoreceptors are also found in flatworms. Annelids possess various sensory receptors including simple eyes, present in the epidermis. In arthropods, compound eyes are found in addition to simple eyes. Statocysts for equilibrium; tactile receptors and chemoreceptors are common in arthropods and molluscs.
Characteristic # 20. Varied Modes of Reproduction:
In invertebrates, the Arthopods have compound eyes, modes of reproduction vary from simple asexual binary fission to most complicated sexual reproduction. In certain cases, parthenogenesis has also been observed in which an unfertilized egg develops into a complete individual. It occurs in rotifers, bees, some other insects and certain crustaceans.
In sexually reproducing invertebrates, hermaphrodites or bisexual forms are found, particularly in coelenterates, platyhelminthes, annelids and crustaceans. Fertilisation is either external or internal. Development is direct or indirect. In the indirect development, the development includes both larval stages and metamorphosis.
Characteristic # 21. Cold-Blooded Animals:
All invertebrates are cold-blooded, i.e., they cannot keep body temperature constant all the time.