Any of the more than 1,100 species of tiny, free-living invertebrates that make up the phylum Tardigrada are known as tardigrades. Other names for them include water bears and moss piglets. They are thought to be the kin of arthropods (such as insects and crustaceans). The majority of tardigrades are 1 mm (0.04 inch) or smaller in size. Around the world, they can be found in a wide range of habitats, including wet moss, flowering plants, sand, fresh water, and the ocean. Numerous genera and species have evolved as a result of adaptation to this wide variety of environmental factors.

Tardigrades have a well-developed head region and a small body made up of four fused segments, each of which has two short, sturdy, independent limbs that are typically terminated by a number of sharp claws. The creatures have There are no known specific organs of circulation or respiration; the fluid that transports blood and oxygen (which diffuses through the animal’s integument and is stored in cells within the hemocoel) is contained in the tardigrade’s body cavity (hemocoel). The entire alimentary canal runs the length of the body. The majority of tardigrades that consume plants do so by using their stylets, which are structures that resemble spears and are located close to the mouth. A few tardigrade species are carnivorous predators. When it comes to reproduction, tardigrades have two options: sexual reproduction or asexual reproduction (parthenogenesis or hermaphroditism). Either the anterior end of the alimentary canal or the external directly through an aperture in front of the anus are used for the release of eggs.

The tardigrades’ amazing resistance to desiccation and extremely low temperatures is perhaps their most surprising trait. The body dries out and looks likelifeless ball (or tun) when they enter the “tun” stage of suspended animation, which occurs under unfavorable circumstances. In this condition, their metabolism could slow to just 0.01 percent of what it would normally be. For years or even decades, tardigrades can endure as tuns to wait out dry circumstances. Additionally, when brought back to normal room temperature after spending eight days in a vacuum, three days in helium gas at ambient temperature, and several hours at a temperature of 272 °C (458 °F), the specimens sprang to life once more 60% of specimens placed in liquid air after 21 months at a temperature of 190 °C (310 °F) also came back to life. In the tun form, tardigrades are easily spread by wind and water.


Any member of the invertebrate phylum Echiura, also known as Echiuroidea or Echiurida, is referred to as a spoonworm or echiurid. Almost all spoonworms are marine-only creatures. They have a sausage-like structure and a flattened “head” extension that is curved along its lateral edges and occasionally shaped like a scoop or spoon to generate an anterior proboscis that is not retractable and is very muscular. Food collecting is done through the proboscis. The size of adult spoonworms ranges from a few millimeters to 600 millimeters (2 feet). In Urechis, the proboscis is only a small portion of the length of the trunk; in Ikeda, it can be up to ten times as long as the trunk. There have been over 150 species described.
All around the planet, seabeds contain spoonworms. While some species live in rock fissures, the majority reside in mud-filled tunnels. Some species can even be found in brackish water. The body wall muscles contract peristaltically to keep water flowing through the burrow. Although there is likely some cross-body respiration, the hind intestine is frequently used as a respiratory organ by pushing water into and out of the anus.

Tardigrade photos

Life cycle

Aside from individuals belonging to the Bonelliidae family, the sexes are distinct and outwardly similar. The coelomic fluid present in the bodily cavity provides nutrition to ova or spermatozoa produced by an unpaired gonad until maturation is finished. The anterior nephridia’s ciliated funnels, which stop serving as excretory organs, pick up the mature gametes. The gametes are then transported into nephridial sacs with thin walls, where they are kept until they are released into the ocean, where fertilization occurs. The zygote undergoes spiral cleavage; a free-swimming trochophore larva grows and transforms into a burrowing worm.
The male of some Bonelliidae species, which barely measure 1 to 6 mm (0.04 to 0.24 in.) in length, lives as a parasite inside a remodeled nephridium (or female’s uterus). A female’s proboscis, which secretes a hormone that impacts larval development, comes into contact with a larva to cause the creation of a male. The larva enters the esophagus and transforms into a sexually mature male with a diminished metamorphosis; nevertheless, the proboscis and circulatory system do not form, leaving merely a diminished digestive system. Although the entire body is covered in cilia (like the larva), the gonoduct system fully develops. In order to possibly fertilize the eggs when they are shed into the sea, the male stays inside the female’s egg-storage organ.

Form and function

The body wall of the spoonworm’s sac-like trunk is made up of three layers of muscle. The muscular but noncontractile tactile proboscis is located at the front of the body and is extremely movable and capable of significant expansion in some species. The animal’s brain is located in its proboscis. At the tip of the proboscis is where the mouth is located. A pair of setae and one or two setae circles are typically present on the ventral surface behind the mouth and the anus, which is situated posteriorly. Coelomic fluid, which comprises two types of cells and, in some species, one type of cell that contains hemoglobin, fills the large body cavity. The anus opens posteriorly, and the alimentary canal is coiled. There develops a pair of excretory anal vesicles from the intestine’s tail. Nephridia ranging from one to several are attached to the ventral body wall. The majority of animals have a closed circulatory system made up of blood vessels in the dorsal, ventral, and neurointestinal regions. The ventral nerve cord.
No one has real gonads. Instead, the coelomic lining from which the eggs or sperm are produced is lost into the coelomic fluid. The gametes are then released through the nephridia from that point.
The size of the proboscis affects the different feeding techniques. Animals with lengthy proboscises reach out and collect debris from the burrow entrance; the debris is then carried to the mouth linked to strings of mucus. A mucous tube is typically secreted and forms the lining of the burrow in species with small proboscises. Food particles are adhered to the mucous tube when they are drawn into the burrow with the water flow for breathing. The tube and its contents are inhaled after this has been filled. Urechis species, on the other hand, eat in suspension, gathering food particles on a mucous net that they exude over the channel of their burrow.


Originally associated with the peanut worms (sipunculids), spoonworms were later placed in the phylum Annelida by Adam Sedgewick. They were not usually recognised as Annelida, and they were recognized as a different phylum after W.W. Newby (1940) researched the embryology and development of Urechis caupo. Even while spoonworms and annelids do not exhibit any signs of segmentation as adults, molecular research from the late 1990s suggests that they may have a similar ancestor. They are now (tentatively) included as a class within the phylum Annelida by certain sources.


any of the diverse group of marine invertebrates known as Ctenophora. The name of the phylum comes from the vertical rows of ciliary combs that cover the animal’s surface (from the Greek ctene, or “comb,” and phora, or “bearer”). The body shape is comparable to the cnidarian medusa. Other common names for different ctenophores include sea walnuts, sea gooseberries, and cat’s eyes.
Comb jellies are typically tiny in size, although at least one species, the Venus’s Girdle, can grow to be more than one meter (3 feet) long. One parasitic species has a diameter of just 3 mm (1/8 inch). All ctenophores are restricted to marine areas, though some do live in somewhat brackish water. They are found practically everywhere in the ocean, especially near the surface waters close to coast. At least two species (Pleurobrachia pileus and Beroe cucumis) have a global range, although the majority do not. Ctenophores float freely suspended in the water, with the exception of a few crawling and parasitic species. Particularly in bays, lagoons, and other coastal areas, they are regularly carried into enormous swarms. They all feed a variety of tiny planktonic organisms, with the exception of one parasite species. When present in large numbers, ctenophores eat the majority of fish fry, larval clams, crabs, and oysters, along with copepods and other planktonic creatures that would otherwise be used as food by fish used for human consumption like sardines and herring. However, certain fish also eat comb jellies, so the cycle continues.

Natural history

Because they are hermaphrodites, ctenophores develop their eggs and sperm (gametes) in distinct gonads along the meridional canals that support their comb rows. Most ctenophores release these gametes into the water, where they are fertilized and grow into embryos. In Pleurobrachia and other Cydippida, the larva and adult are nearly identical, therefore maturation doesn’t involve much change. The so-called cydippid larva, which is ovoid or spherical with two retractable tentacles, is present in the majority of ctenophores, though. The transformation of the globular cydippid larva into an adult occurs instantly in ovoid-shaped adults, although it takes members of flattened groups a little longer. The only other organism with a free-swimming planula larva analogous to the cnidarians is the parasite Gastrodes.

Form and function

Although Beroe cucumis is pink and the Venus’s girdle (Cestum veneris) is a beautiful violet, most ctenophores are colorless. When floating in water, the colorless species are transparent save for their exquisitely iridescent rows of comb plates. The majority of comb jellies are bioluminescent, and at night they put on some of the most vivid and stunning animal world displays of bluish or greenish light.
The majority of the almost 90 species of comb jellies that are known are spherical or oval in shape, with a mouth at one end (oral) and a prominent sensing organ (the statocyst) at the other. The eight comb rows that project orally from the statocyst serve as the animals’ means of propulsion. Each comb row consists of consisting of combs, which are a collection of transverse plates with very big cilia joined at the base. The animal often swims oral end first because when the cilia beat, the effective stroke is in the direction of the statocyst. The more ape-like animals (order Cydippida) have two long, branching tentacles that can be retracted to help them catch prey. Colloblasts, a type of sticky cell found solely in ctenophores, are abundantly present in the tentacles. Prey organisms attach to the sticky secretion produced by these cells upon contact.


Phylum Coelenterata used to group ctenophores and cnidarians together. The following ctenophore traits, however, have led modern authorities to distinguish between cnidarians and ctenophores: (1) the absence of the stinging cells (nematocysts) that are distinctive of cnidarians; (2) the presence of a distinct mesoderm in the ctenophores; (3) fundamental differences in embryological development between the two groups; and (4) the biradial symmetry of ctenophores. However, it is commonly accepted that cnidarians and ctenophores have a shared evolutionary ancestor


Any member of the phylum of wormlike marine invertebrates known as the Hemichordata, which includes chordates and echinoderms, is referred to as a hemichordate.Due to the buccal diverticulum’s resemblance to a primitive notochord—the dorsal, or back-side, supporting axis of the more complex notochord, the tubular protrusion from the mouth cavity into the proboscis, or “snout,” primitive vertebrates—the term Hemichordata—from At first, the term “half-chordate”—derived from the Greek words hemi, which means “half,” and chorde, which means “string,” was offered. However, it has recently been discovered that the diverticulum differs significantly from the vertebrate notochord in terms of its origin and function, casting doubt on this notion. Hemicordates are believed to be more closely linked to echinoderms than to chordates, according to molecular analyses. There have been descriptions of about 90 species.
There are three classes in the Hemichordata: Enteropneusta, Planctosphaeroidea and Pterobranchia. Acorn worms, also known as enteropneusts (about 70 species), are solitary, worm-like, bilaterally symmetrical creatures that can have beautiful colors. Because of the way the proboscis and collar look, they are also referred to as acorn worms. About 20 kinds of tiny, colonial, tube-building organisms are known as pterobranchs. Several floating larvae are all that are known of the Planctosphaeroidea. From the White Sea and Greenland south to New Zealand and the Cape of Good Hope, enteropneusts are frequent in the intertidal zones; they can be found offshore at depths of at least 400 meters (approximately 1,300 ft). They range in length from a few centimeters to two meters (Saccoglossus pygmaeus of the North Sea).



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