An invertebrate is any animal without a backbone. Invertebrates make up 95% of all species of animals on the earth, and the variety of invertebrates is enormous. Scientists group or “clas sify” all of these different types of animals into broad categories called phyla, on the basis of their patterns of symmetry and on the basis of their overall body plan.
Invertebrates are classified as 9 groups. Those are...
i. Protozoa:
Protozoa are one-celled animals found worldwide in most habitats. Most species are free living, but all higher animals are infected with one or more species of protozoa. Infections range from asymptomatic to life threatening, depending on the species and strain of the parasite and the resistance of the host. More than 50,000 species have been described, most of which are free-living organisms; protozoa are found in almost every possible habitat. The fossil record in the form of shells in sedimentary rocks shows that protozoa were present in the Pre-cambrian era.
"Anton van Leeuwenhoek" was the first person to see protozoa, using microscopes he constructed with simple lenses. Protozoan diseases range from very mild to life-threatening.
Individuals whose defenses are able to control but not eliminate a parasitic infection become carriers and constitute a source of infection for others.
Examples of Protozoa:
* Cryptosporidium
* Toxoplasma gondii
* Pneumocystis carinii
* Acanthamoeba
* Giardia lamblia
* Naegleria.
Structure:
Protozoa are microscopic unicellular eukaryotes that have a relatively complex internal structure and carry out complex metabolic activities. Most parasitic protozoa in humans are less than 50 μm in size. The smallest are 1 to 10 μm long, but Balantidium coli may measure 150 μm. Protozoa are unicellular eukaryotes. As in all eukaryotes, the nucleus is enclosed in a membrane. The organelles of protozoa have functions similar to the organs of higher animals.
The plasma membrane enclosing the cytoplasm also covers the projecting locomotory structures such as pseudopodia, cilia, and flagella. The outer surface layer of some protozoa, termed a pellicle, is sufficiently rigid to maintain a distinctive shape, as in the trypanosomes and Giardia. However, these organisms can readily twist and bend when moving through their environment.
Life cycle Stages:
Some protozoa have life phases alternating between proliferative stages (e.g., trophozoites) and dormant cysts. As cysts, protozoa can survive harsh conditions, such as exposure to extreme temperatures or harmful chemicals, or long periods without access to nutrients, water, or oxygen for a period of time. Being a cyst enables parasitic species to survive outside of a host, and allows their transmission from one host to another. When protozoa are in the form of trophozoites they actively feed. The conversion of a trophozoite to cyst form is known as encystation, while the process of transforming back into a trophozoite is known as excystation. Protozoa reproduce asexually by binary fission or multiple fission. Many protozoan species exchange genetic material by sexual means (typically, through conjugation); however, sexuality is generally decoupled from the process of reproduction, and does not immediately result in increased population.
Diseases:
Some protozoa are human parasites. They causing diseases including
* Malaria, Amoebiasis
* Giardiasis
* Toxoplasmosis
* Cryptosporidiosis
* Trichomoniasis
* Chagas disease
* Leishmaniasis
* Sleeping Sickness
* Amoebic dysentery
* Acanthamoeba Keratitis
* Primary Amoebic Meningoencephalitis
Some protozoa are animal parasites.The protozoan Ophryocystis elektroscirrha is a parasite of butterflies. It infects the butterfly entering the larval stage. The spores are found on the body of infected butterflies. These spores are passed, from female to caterpillar. Severely infected individuals are weak, unable to expand their wings, or unable to eclose, and have shortened lifespans, but parasite levels vary in populations. This is not the case in laboratory or commercial rearing, where after a few generations, all individuals can be infected. Infection with this parasite creates an effect known as culling whereby infected migrating animals are less likely to complete the migration. This results in populations with lower parasite loads at the end of the migration.
ii. Porifera:
Poriferans are commonly referred to as sponges. An early branching event in the history of animals separated the poriferas from other metazoans. As one would expect based on their phylogenetic position, fossil poriferas are among the oldest known animal fossils, dating from the Late Precambrian. Since then, poriferas have been conspicuous members of many fossil communities; the number of described fossil genera exceeds 900. The approximately 5,000 living porifera species are classified in the phylum Porifera, which is composed of three distinct groups, the Hexactinellida (glass poriferas), the Demospongia, and the Calcarea (calcareous poriferas).
Poriferas are characterized by the possession of a feeding system unique among animals. Poriferans don't have mouths; instead, they have tiny pores in their outer walls through which water is drawn. Cells in the porifera walls filter goodies from the water as the water is pumped through the body and out other larger openings. The flow of water through the porifera is unidirectional, driven by the beating of flagella which line the surface of chambers connected by a series of canals. Porifera cells perform a variety of bodily functions and appear to be more independent of each other than are the cells of other animals.
Characteristics of Porifera:
Some common characteristic of poriferas is that they have needlelike structure called spicules which can barely be found in their soft body wall. The spicules serve as the skeleton of poriferas for it supports the body of spongy. They also secrete unpleasant sensation when you press the body of porifera just enough to prick your by spicules. Poriferas are black and smooth when alive that is why they are hard to identify when they are still alive. You can also see that bath poriferas abound in some beaches. If you want their porifera all you need to do is to wash it and bleach it under the sun, the porifera dies and leaves the familiar skeleton that we commonly known as a porifera. As you can see in bath porifera, it consists of fibers which are made of porifera. It is the reason why the porifera retains its shape and also the counterpart of spicules in diverse poriferas. In some poriferas you will see the external view of the porifera and longitudinal section of the porifera. They have a longitudinal section that shows a large central cavity called gastral cavity. Poriferas have a dark inner layer of the body wall represents a layer of special cells which have flagella and flagellum for single-celled.
The special cells which line the inner surface of the porifera’s body wall are known as collar cells. They are called collar cells because it is inside the collar cells that the food of the porifera is digested. The flagella of the collar cells beat constantly. This draws water into the gastral cavity through the body pores and out through the opening at the top. The water current brings small food particles into the gastral cavity. This manner of food getting is important to the stationary adult porifera which cannot move about to hunt for its food.
Importance of Porifera:
* It is use to maintain the most in agriculture like in radish and some fast-growing seeds.
* Moistened sponges are put inside a container and closed by an inverted glass and the sponge creates a moist surrounding that makes the seed sprout.
* It is wrap in some tools for you to hold it comfortably and so that it will not slip on your bare hands.
* It is also put to in some boxes with fragile things to protect and to avoid cracks for it offers soft and safety for it contour the shapes of breakable.
iii. Coelenterate:
Aristotle the father of biology first called the coelenterates as "cnidae" because of their stinging qualities. He considered them as intermediate organisms between plants and animals. In 1723 Peyssonel established their animal nature. In 1774 "Trembly" described "Hydras" Linnaeus, Lamarck and Cuvier placed coelenterates in animal group Radiata. In 1847 Leuckart Coined the word coelenterata. He included sponges and ctenophores also in it.
Coelenterata is an obsolete term encompassing two animal phyla, the Ctenophora (comb jellies) and the Cnidaria (coral animals, true jellies, sea anemones, sea pens, and their allies). They have very simple tissue organization, with only two layers of cells, external and internal and radial symmetry. Some of the examples are corals, sea anemone which are colonial and hydra, jelly fish which are solitary.
Characteristics:
All coelenterates are aquatic, mostly marine. The bodyform is radially symmetrical with a tissue grade of organisation. The body has a single opening hypostome surrounded by sensory tentacles leading into a spacious cavity called the gastrovascular cavity or coelenteron. Digestion is both intracellular and extracellular. Respiration and excretion are accomplished by simple diffusion. A network of nerves is spread throughout the body. Many forms exhibit polymorphism, wherein different types of individuals are present in a colony for different functions. These individuals are called Zooids. These animals generally reproduce asexually by budding, though sexual reproduction does occur in some groups.
iv. Platy helminthes:
The simplest animals that are bilaterally symmetrical and triploblastic (composed of three fundamental cell layers) are the Platyhelminthes, the flatworms. Flatworms have no body cavity other than the gut and lack an anus; the same pharyngeal opening both takes in food and expels waste. Because of the lack of any other body cavity, in larger flatworms the gut is often very highly branched in order to transport food to all parts of the body. The lack of a cavity also constrains flatworms to be flat; they must respire by diffusion, and no cell can be too far from the outside, making a flattened shape necessary.
Flatworms were once divided into three groups. The mostly free-living Turbellaria include the planarian, Dugesia, these are found in the oceans, in fresh water, and in moist terrestrial habitats, and a few are parasitic. The Trematoda, or flukes, are all parasitic, and have complex life cycles specialized for parasitism in animal tissues. Members of one major taxon of flukes, the Digenea - which includes the human lung fluke depicted at right - pass through a number of juvenile stages that are parasitic in one, two, or more intermediate hosts before reaching adulthood, at which time they parasitize a definitive host. The Cestoda, or tapeworms, are intestinal parasites in vertebrates, and they also show anatomical and life history modifications for parasitism.
It now seems likely that the first two of these groups are paraphyletic; that is, they contain some but not all descendants of a common ancestor. Recent molecular studies suggest that the Platyhelminthes as a whole may even be polyphyletic, having arisen as two independent groups from different ancestral groups. If this latter view is correct, then most of the flatworms may belong to the Lophotrochozoa, a large group within the animal kingdom that includes molluscs and earthworms, while the rest belong near the base of animal diversity.
Characteristics of Platy Helminthes:
* Free-living and parasitic unsegmented worms that are dorsoventrally flattened (depressed)
* Triploblastic, acoelomoate, bilaterally symmetrical
* Spiral cleavage
* Complex gut, but still incomplete; gut reduced to absent in parasitic species
* Cephalization
* Organ/organ system level of organization
* Protonephridia as excretory/ osmoregulatory structures
* Hermaphroditic; complex reproductive system
v. Nemathelminthes:
They are also commonly called roundworm. Their bodies are round but non segmented. They are endoparasite generally. They are generally unisexual animals. Their mouth is provided with suckers. Some are found inside the body of human beings. Generally these animals completes life cycle in two hosts.More than 12000 species are known till now.
Characteristics:
* They are marine or freshwater animal.
* They are pseudocoelomate which means they have a cavity called pseudocoel between the gut and body wall.
* They are non segmented round worm.
* Their body is covered with cuticle.
* They have simple type of digestive system.
* They do not have respiratory system and circulatory system.
* They have poorly developed nervous system.
* They are unisexual animals.
* They are endoparasite
* The mouth is provided with hooks and suckers.
vi. Annelid:
The annelids also known as the ringed worms or segmented worms, are a large invertebrate phylum, with over 17,000 modern species including ragworms, earthworms, and leeches. Various forms specialise in their respective ecologies; some in marine environments as distinct as tidal zones and hydrothermal vents, others in fresh water, and yet others in moist terrestrial environments.
The annelids are bilaterally symmetrical, triploblastic, coelomate organisms. They also have parapodia for locomotion. Although most textbooks still use the traditional division into polychaetes, oligochaetes (which include earthworms) and leech-like species, research since 1997 has radically changed this scheme, viewing leeches as a sub-group of oligochaetes and oligochaetes as a sub-group of polychaetes. In addition, the Pogonophora, Echiura and Sipuncula, previously regarded as separate phyla, are now regarded as sub-groups of polychaetes. Annelids are considered members of the Lophotrochozoa, a "super-phylum" of protostomes that also includes molluscs, brachiopods, flatworms and nemerteans.
There are over 22,000 living annelid species, ranging in size from microscopic to the Australian giant Gippsland earthworm and Amynthas mekongianus, which can both grow up to 3 metres (9.8 ft) long.
Sub Groups of Annelid:
* Polychaetes:
As their name suggests, they have multiple chetae per segment. Polychaetes have parapodia that function as limbs, and nuchal organs that are thought to be chemosensors. Most are marine animals, although a few species live in fresh water and even fewer on land.
* Clitellates:
These have few or no chetae per segment, and no nuchal organs or parapodia. However, they have a unique reproductive organ, the ring-shaped clitellum around their bodies, which produces a cocoon that stores and nourishes fertilized eggs until they hatch or, in moniligastrids, yolky eggs that provide nutrition for the embyros.
The clitellates are sub-divided into:
Oligochaetes: Which includes earthworms. Oligochaetes have a sticky pad in the roof of the mouth. Most are burrowers that feed on wholly or partly decomposed organic materials.
Hirudinea:
Whose name means "leech-shaped" and whose best known members are leeches. Marine species are mostly blood-sucking parasites, mainly on fish, while most freshwater species are predators. They have suckers at both ends of their bodies, and use these to move rather like inchworms.
vii. Arthropoda:
Arthropoda, largest and most diverse animal phylum. The arthropods include crustaceans, insects, centipedes, millipedes, spiders, scorpions, and the extinct trilobites. Arthropods are characterized by a segmented body covered by a jointed external skeleton (exoskeleton), with paired jointed appendages on each segment; a complex nervous system with a dorsal brain, connective nerves passing around the anterior end of the digestive tract, and a ventral nerve cord with a ganglion in each body segment; an open circulatory system with a dorsal heart into which blood flows through paired openings (ostia); and a greatly reduced body cavity (coelom). Because the jointed exoskeleton blocks growth of the organism, it must be shed periodically. This phenomenon, called molting, or ecdysis, is a characteristic feature of the phylum; it permits rapid growth in size and significant change in body form until the new exoskeleton, secreted by the animal, has hardened. Arthropods are mainly terrestrial, but aquatic representatives are well known.
Characteristics of Arthropoda:
* Exoskeleton:
Arthropods are invertebrates, which means their bodies do not have internal bones for support. To compensate for this, they produce a hard exoskeleton made of chitin, a 
mixture of lipids, carbohydrates and protein, which covers and protects their bodies like a suit of armor. As arthropods grow, they must shed or molt their exoskeletons. They first produce new, softer exoskeletons underneath the old ones. Once their hardened, old coverings crack and shed, they sport roomier, albeit soft, exoskeletons. Arthropods are incredibly vulnerable during the molting process, and will often hide until their new exoskeletons harden.
* Segmented Bodies:
Arthropods have bodies that are internally and externally segmented. The number of segments depends on the individual species; millipedes, for example, have more segments than lobsters.
* Jointed Appendages:
The name arthropod actually comes from the Greek "arthro," meaning joint, and "pod," meaning foot. All arthropods have jointed limbs attached to their hard exoskeletons that allow for flexibility and movement. The joints generally bend in only one direction but allow for sufficient predatory and defensive actions.
* Open Circulatory System:
An arthropod has an open circulatory system. This means instead of a closed circulatory system of interconnected veins and capillaries, an arthropod’s blood is pumped through open spaces called sinuses in order to reach tissues. An arthropod does, however, have a heart which pumps blood into the hemocoel, the cavity where the organs are located, where it surrounds the organs and tissues.
viii. Mollusca:
The most universal features of the body structure of molluscs are a mantle with a significant cavity used for breathing and excretion, and the organization of the nervous system. The most abundant metallic element in molluscs is calcium.
Molluscs have developed such a varied range of body structures, it is difficult to find synapomorphies (defining characteristics) to apply to all modern groups. The most general characteristic of molluscs is they are unsegmented and bilaterally symmetrical.
The molluscs compose the large phylum of invertebrate animals known as the Mollusca. Around 85,000 extant species of molluscs are recognized. Molluscs are the largest marine phylum, comprising about 23% of all the named marine organisms. Numerous molluscs also live in freshwater and terrestrial habitats. They are highly diverse, not just in size and in anatomical structure, but also in behaviour and in habitat. The phylum is typically divided into 9 or 10 taxonomic classes, of which two are entirely extinct. Cephalopod molluscs, such as squid, cuttlefish and octopus, are among the most neurologically advanced of all invertebrates and either the giant squid or the colossal squid is the largest known invertebrate species. The gastropods are by far the most numerous molluscs in terms of classified species, and account for 80% of the total. The scientific study of molluscs is called malacology.
The three most universal features defining modern molluscs are a mantle with a significant cavity used for breathing and excretion, the presence of a radula, and the structure of the nervous system. Other than these things, molluscs express great morphological diversity, so many textbooks base their descriptions on a "hypothetical ancestral mollusc". This has a single, "limpet-like" shell on top, which is made of proteins and chitin reinforced with calcium carbonate, and is secreted by a mantle covering the whole upper surface. The underside of the animal consists of a single muscular "foot".
ix. Echinodermata:
The echinoderms are an edge of animals deuterostomos exclusively marine and benthic. Its name alludes to its exclusive skeleton consisting of ossicles internal calcareous. Possess symmetry pentarradial secondary, unique in the animal kingdom, and a characteristic water vascular system. There are approximately 7,000 living species one more 13,000 that are already extinct, since their history dates back to the early Cambrian, 2 being one of the best animal groups represented on the record fossil. Echinoderms are known since antiquity. They appear in fresh Cretan over 4,000 years, but were not recognized as taxon independent until 1847.
Characteristics:
* Symmetry:
Originally echinoderms are animals with radial symmetry, less larvae that have bilateral symmetry. Thus, the body is divided into five regions that are arranged around a central disk. As a result of this symmetry the head not unlike the rest of the body rather than the madreporica plate, we indicated the true axis of symmetry, since it is located in one of the 5 sections. In the course of evolution some groups have returned to a rough or strongly bilateral symmetry.
* Integument:
Under the epidermis is one dermis derived from mesoderm containing the skeletal elements called ossicles, and then there muscle layers and the peritoneum of the coelom. The degree of development of these elements varies among groups in sea urchins ossicles are strongly linked together to form a rigid shell and, consequently, the muscles of the body wall are underdeveloped, while in the sea cucumbers ossicles are tiny and are dispersed in the fleshy dermis, well-developed muscle layers being.
The ossicles are composed of calcium carbonate as calcite with small amounts of magnesium carbonate. Often they have projections (tubers, granules) and fixed or mobile spines.
The asteroids and echinoids also have an exclusive structures shaped clamp, called pedicellariae, which have different functions: remove debris and larvae that try to bind to the body, defend the animal from predators or participate in capturing prey.
* Celoma vascular system and aquifer:
The coelom of echinoderms develops as a tripartite system forming protoceles, mesoceles and metaceles pairs. In adults, the celoma derived from embryonic metacele and forms the lining of the gonads, the perivisceral celoma and the water vascular system or ambulacral apparatus.
* Circulation and excretion:
The blood vessels are in connection with lagoons or breasts, therefore, is a circulatory system open; they lack heart. Excretory organs are underdeveloped; dissolved substances are eliminated through the water vascular system therefore podiums or the madreporite.
