Invertebrates

Invertebrates are creatures which lack a backbone. The word "invertebrate" refers to lacking a spinal cord or not vertebrate. Invertebrates constitute around 97% of all the creatures present on the Earth. 

The List of Invertebrates

1. Porifera- sponges

2. Cnidaria

1. sea anemones

2. hydra

3. corals

4. jelly-fish

3. Platyhelminthes-flatworms

1. flukes

2. tapeworms

4. Nematoda-roundworms

1. Trichinella

2. Ascaris

3. hookworms

4. pinworms

5. Rotifera--rotifers

6. Annelida-segmented worms

1. earthworm

2. leeches

7.  Mollusca-clams, oysters, snails, and octopus

8. Arthropoda

1. Subphylum: Trilobita – Trilobites (extinct)

2. Subphylum: Chelicerata - Horseshoe crabs, spiders, scorpions, mites, & ticks

3. Subphylum: Mandibulata - Crustaceans, insects, millipedes, centipedes

9. Echinodermata: starfish, sea cucumbers, sea lilies

Porifera- sponges: 

Phylum Porifera consists of sponges. There are around 800 different types of sponges. Most of which are marine. Freshwater fishes are filter feeders they use their body to trap any minute fishes. They are sessile. They are asymmetrically shaped and are made up of a protein called sponging and hard filaments called spicules which are made from calcium carbonate and silicon dioxide. The body of sponges has pores or gaps through which water enters their bodies. Sponges have a cellular level of organization.Choanocytes are cells that line the pores in a sponge and have a flagellum that twists to pull in water. Amebocytes are cells responsible for delivering the cells to every part of the sponge's body. Excess of water in the body leaves the body through the osculum. Sponges reproduce both sexually and asexually.

Sponges reproduce asexually by 

1. external or internal buds

2. Fragmentation

Sponges can reproduce sexually. The male sponge releases the sperm into the water which enters the body of the female sperm. Fertilization takes place inside the body of the female sponge after which larvae are released to the water.

Cnidaria

Phylum Cnidaria includes sea anemones, jellyfishes, hydra and corals. Except for hydra (freshwater organism), all the other cnidarians are marine. They have a spiral symmetry and have appendages that help them meet their nourishment. Cnidarians display two body shapes:

1. Polyp: They are sessile. They have arms and mouths at the top.

2. Medusa: They are motile with arms and mouth at the base.

Cnidarians may exist in both the forms at different stages of their life cycle or may exist in one of these two phases. Cnidarians have an empty gastrovascular pit which is fixed within the gastrodermis. Epidermis covers the outside of the cnidarian and jelly-like material called mesoglea is present between the layers. The special structure of the cnidarians is their stinging structure.

Platyhelminthes (Flatworms)

One main feature of phylum Platyhelminthes is that they are dorsoventrally smoothed and flattened. They have a definite posterior end and anterior end. This gives them a bilateral symmetry. Their bodies are rather strong and hence they are said to be acoelomates. A few flatworms are parasites. Others exist as free-living scroungers or carnivores. Examples of such flatworms are flukes and tapeworms. Flatworms have just a mouth for both nourishment and discharging wastes. Their sensory system is formed by a nerve net. They have eyespots that are light-sensitive at the foremost end. They also have particular flame cells to dispose of the waste. The most widely recognized free-living flatworm found in water or wet spots in the planarian. They are bisexuals.

Tapeworms are partitioned into areas called proglottids. The proglottids each have an entire reproductive framework which creates fertilized eggs. Tapeworms are bisexual. Tapeworm’s front end is known as the scolex. It is adjusted with both snares and suckers so that it joins to the host's digestive tracts. Tapeworm eggs are known to withstand bubbling water.

Flukes have complex life cycles and it includes multiple hosts. Schistosomiasis is an ailment that affects the kidneys and liver which is caused by a fluke. No cure has been discovered yet.

Nematoda (Roundworms)

Roundworms are barrel-shaped organisms. Their size varies from minute to 20 inches long. Roundworms are pseudocoelomates; they have a body pit that is not totally lined.

Ascaris (roundworm)

The pseudocoel plays the role of a hydrostatic skeleton. This helps in the contraction of the muscles. Roundworms have a total gut which sets them apart from flatworms. This makes them ready to process nourishment. Roundworms are devoid of blood or heart and all the supplements are circulated by a non-blood liquid.

Majority of the roundworms are parasites and are seen in all natural environments. They are symmetrical and also unsegmented. In spite of the fact that they are hollow and round, they form for the most part tapering at both the ends of their body. They are protected with a broad defensive fingernail skin that is flexible and also that can be shed. They have isolate genders and reproduce sexually.

Trichinosis is caused by the roundworm Trichinella. It enters the muscles and leaves calcium stores which causes muscle compression. Trichinosis is also known to influence the heart. Ascaris parasitizes the human lungs while the Filaria worm attacks the lymphatic framework creating swelling.

Rotifer

The phylum Rotifera have a crown of cilia at the head-enveloping their mouth for development and eating. Their bodies are protected with an external layer of chitin. Having separate genders, their reproduction is sexual. A few animal categories contain just females. They reproduce by parthenogenesis.

Mollusks

The phylum Mollusca consists of bivalves, octopus, snails and slugs, squid, and the chambered nautilus. They generally have sturdy limestone shells and are found in all natural surroundings. Mollusks have bilateral symmetry. Mollusks inhale through gills or lungs which are situated underneath a defensive layer called the mantle. All mollusks with a peculiar exception of bivalves possess a rasping, tongue-like radula for scratching sustenance. The circulatory framework includes a three-chambered heart and open-streaming framework but octopuses and squids are exceptions.

Gastropods have a foot on their stomach. Snails are a case of gastropod. When a snail does not have a shell it is called a slug. Snails and slugs stroll on their paunch. Marine snails have gills whereas land snails also called pulmonate snails have an air gap for relaxing.

The class of mollusk called Bivalvia consists of shellfish, clams, mussels, and scallops. These mollusks have two shells pivoted together by a tendon and solid adductor muscles are used to open and close the shells. They have gills. They move by stream drive.

Cephalopods have a foot on their head like octopus, squid, and nautilus. Most cephalopods possess mouths, arms, and jaws and are dynamic predators. They are in need of external shells apart from the Nautilus. Octopus's shell exits within their body.

Annelids (Segmented Worms)

Annelid Polychaetes

The phylum Annelida consists of the segmented worms and is plentiful in the environment... External segments can be seen as a ring-like structure along the body. Segmentation gives worms more adaptability in terms of development and movement. Annelids have a tube inside their tube body arrangement known as a Coelom. It is completely lined and contains the body organs. Annelids are known to have bilateral symmetry, a developed mind and various sense organs demonstrating cephalization. The hydrostatic skeleton is present in the form of a coelomic liquid,

Worms have a place in phylum Annelida. Every segment of the earthworm has setae which are external abounds made of chitin. These help the earthworm to move and to tunnel into the soil. Earthworms are known to have a head and a central sensory system. Earthworms breathe through their moist skin.

Bloodsuckers or leeches are also present in the phylum Annelida. Most parasites live in water and suckers are present at both finishes of their bodies.

Arthropods

Phylum Arthropoda members have joined appendages. Arthropods are known to have these attributes:

1. A hard exoskeleton. It is normally made out of chitin.

2. they experience occasional ecdysis when they shed their exoskeleton

3. they have specific body segments like head, thorax, cephalothorax, and midriff

4. Jointed extremities, for example, mouthparts, legs, and receiving the wire.

5. open circulatory framework

Echinoderms

The phylum Echinodermata consists of the starfish, sea urchins, and sea cucumber. They are the most advanced invertebrates. Echinoderms’ endoskeleton is made out of settled calcium plates called ossicles is their special feature. They have a water vascular structure. Water enters through an opening called the madreporite directly into a short stone channel into the ring trench. Outspread canals connect with the ring channel and make up the five-area symmetry. They replicate agamically by a discontinuity or also sexually with outside fertilization.

Feather Starfish

Starfish fall in the class Asteroidea. They are dynamic marine predators with five arms set off from a focal circle. Ocean urchins and sand dollars are part of the class Echinoidea which use the spines of their skin and tube feet for movement. The class Crinoidea comprises ocean lilies and plume stars with defined spread arms around their mouth for channel bolstering. Brittle stars of class Ophiuroidea have thin arms connected to their focal plate and can move swiftly. Ocean cucumbers fall in the class Holothuroidea and are delicate, slug-like life forms with weathered skin.

The Biological Classification Of Invertebrate Animals

All the animal species in our surroundings have skeletons inside their body. A skeleton is the bone structure that provides shape to the animal. The main axle part of the skeleton is the backbone that contains the spine. In biology, it is known as the vertebral column. The name derived for such animals is vertebral animals. These are mostly organisms with a highly developed body. However, several organisms do not possess a skeleton inside their body. Consequently, they do not have any backbone or spine. These animals are known as invertebrates. Arthropods, molluscs, annelids and cnidarians are included under this family. 

In the previous chapters of Biology taxonomy, you must be familiar with these subclasses. You can recall the animals included under arthropods such as insects, arachnids, crustaceans and myriapods. Likewise, the organisms coming under Annelida are earthworms and leeches. Aquatic creatures such as snails, octopuses, squids, bivalves, and chitons are kept under the category of molluscs. The famous sea creatures such as hydras, jellyfishes, corals and sea anemones are the cnidarian group of organisms. Because of the small size of these animals, they are larger in number than all the land and big animals on earth. To give structure to their bodies, some of these animals have fluid-filled hydrostatic skeletons. Exoskeletons are the most commonly found parts in such types of organisms. Different types of shells found in the ocean and sea beaches are great examples of this type of crustacean. All of the animals possess a typical digestive system with differentiated tissues for various functions.