Living Spineless: types of invertebrate animals

A short introduction to animals


With a few exceptions animals generally develop from embryos. Typically a fertilized egg will undergo cleavage (its cells divide without growing). The end result of cleavage is typically a blastula, or a sphere with a single layer of cells that is hollow inside. The blastula then undergoes gastrulation, a process whereby a part of the blastula turns inward, creating a hole called a blastapore. The result of gastrulation is an inner and outer layer of cells: ectoderm which forms the outer skin and the endoderm which forms the digestive cavity. More complex animals continue the folding process until a mesoderm layer arises (this creates a body cavity which holds internal organs).

The sponge most closely resembles the original animal.
The sponge most closely resembles the original animal.
Tiny corals are the tiny builders of massive reefs
Tiny corals are the tiny builders of massive reefs
The tape worm attaches to the intestine with a scolex (picture here) it then absorbed nutrients directly through its skin.
The tape worm attaches to the intestine with a scolex (picture here) it then absorbed nutrients directly through its skin.

Coral Reefs

Invertebrate Animals Group One, Basal animals


The species most like the original animal ancestor are basal animals, and lack true tissues. Eumetazoa (true animals) have specialized cells set apart from other cells by membranes (i.e. they have tissues).

Phylum: Porifera (sponges)

These marine invertebrates are sessile suspension feeders. They draw water through outer pores inside to a vertical cavity. Flagellated cells pick up the particles inside this cavity which are then transferred to the rest of the body. Unlike the other invertebrate groups in this hub the sponge has no nerve cells, or symmetry.

Invertebrate Animals Group Two, Radiata

The Radiata are animals that exhibit radial symmetry, as with all further invertebrate groups, the Radiata are true animals.

Phylum Cnidaria (jellyfish and corals)

Cnidarians (jellies and corals) Cnidarians are mostly marine invertebrates and are distinguished by the presence of Cnidocytes, or poisonous cells. Cnidarians follow two basic body plans: the bell shaped free floating jellyfish or the sessile corals that are rooted in place and flare outwards at the top.The corals are extensive reef builders. These invertebrate animals live off unicellular algae (zooxanthella) and secret calcium shells, creating a diverse habitat for many species. The Cnidarians have not evolved a centralized brain but they are the first of the invertebrate groups to have a nervous system that can coordinate their movement with a neural net. Nor does it have a digestive tube. Thus one body opening must make do for circulation, digestion and excretion.

Invertebrate Animals Group Three: The Bilaterans

The remaining types of invertebrates all show bilateral symmetry and a more complex embryonic development.After gastrulation, which forms the inner and outer layers of cells, bilaterans also develop an additional embryonic layer called mesoderm. In animals like flatworms mesoderm tissue is useful in movement, but most phyla become capable of having a true digestive cavity (made possible by the mesoderm separating the ecto- and endoderm). In more complex animals the mesoderm is also used to attach internal organs.

Phylum: Platyhelminthes: (the flat worms)
Most infamous member: the tapeworm.

Flatworms are the most simple of the bilaterans Their mesoderm tissue does not create a cavity. Without a body cavity they have no lungs for respiration and must receive oxygen via diffusion. To keep all parts of the body ventilated with oxygen the animal needs maximum surface area. This is why flatworms are flat. The need for diffusion restricts flat worms to moist habitats, such as water, leaf litter, and most notoriously as parasites such as the tape worm.

Of the invertebrate groups, the flatworms are the most ancestral animal to display cephalization. This is the formation of a head, and is a logical outgrowth of bilateral symmetry. As bilaterans generally move forward rather than side to side, their eyes need to be facing forward. As the bilateran needs to respond quickly to sensory information, the head is a logical place for a brain. Flatworms have centralized ganglion). However, flatworms do not possess a true gut (with a mouth on one end and an anus at the other) but a gastrovascular cavity.

Fun fact: tapeworms have the most prolific reproduction system of the invertebrate groups and vertebrates as well. The ending segments of the tapeworm are generally entirely eggs.

Phylum: Nematoda: (the roundworms or nematodes)

The roundworms have a mesodermic layer that surrounds the gut and forms its own cavity (a pseudocoelom). This allows a fluid filled cavity around the gut to transport nutrients, making it possible to maintain a round shape. Unlike most flatworms, round worms have a true gut (alimentary canal) that has a distinct mouth and anus. Roundworms are surrounded by a tough cuticle which they must occasionally molt. There is scientific controversy regarding how related the round worms are to arthropods that also molt. Roundworms can be found in soils and decomposing matter but also as plant pests and parasites.

Fun Fact: the tiny and translucent nematode Caenorhabditis elegans or C. elegans is an invertebrate animal that has become a favorite research animal for developmental biology. It compromises roughly a thousand cells which can easily be observed from their original developmental state. This animal is especially useful for studies of brain development (out of the thousand cells that develop, 300 of them are neurons)


The Earthworm is an annelid and literally eats its way through the earth, tilling the soil
The Earthworm is an annelid and literally eats its way through the earth, tilling the soil
The octopus is a mollusk and considered the smartest invertebrate.
The octopus is a mollusk and considered the smartest invertebrate.
The spider belongs to the arthropod family, the most successful of the animal phyla
The spider belongs to the arthropod family, the most successful of the animal phyla

Phylum: Annelida: (the segmented worms).

The body plans of these worms are repetitious segments, which typically have both circular and longitudinal muscles for changing its diameter and length, respectively, this allows the annelid to move in a straight direction without thrashing about like roundworms usually do. The annelids also have a true coelom (a body cavity entirely surrounded by mesoderm tissue). This is essential as annelids have closed circulatory systems (i.e. a heart or hearts pump blood through vessel(s) which is impossible without a mesoderm layer to suspend organs. The most famous annelid is the earthworm, but the leech is nearly as infamous.

Phylum: Mollusca: (the mollusks)

generally the mollusks which include Gastropodia (snails and slugs) Bivalvia (clams and mussels) and Cephalopoda (squids and octopi). Mollusks share the common features of a muscular foot involved in locomotion, a visceral mass containing all the organs, and a mantle layer which secretes a shell (if present). In the gastropods the foot is the bottom muscle layer which moves the rest of the body at a notoriously slow pace. In clams the foot controls the opening and closing of the shell in order to draw in water (the bivalves are suspension feeders). In squids and octopi the foot consists of the tentacles and parts of the head. Mollusks have true coeloms but they have open circulatory systems. This means the circulatory fluid (labeled hemolymph rather than blood) bathes organs directly rather than moving it through vessels (as happens in humans).

Fun fact: mollusks have a visceral body mass that suspends its organs. When this mass comes into contact with salt, it loses its water content via osmosis. This is what makes snails and slugs dissolve after salting them.

Fun Fact: the octopus is considered to be the most intelligent invertebrate.For more info check out my hub on these amazing marine invertebrates

Phylum: Arthropoda (the arthropods)

the arthropods include many members, including Cheliceriformes (such as spiders and scorpions), Myriapoda (centipedes and millipedes), Hexapoda (the insects) andd Crustacea (such as lobsters, crabs, and shrimp). All arthropods have true coeloms, open circulatory systems, and segmented bodies. They also have hard exoskeletons with jointed legs.

The Chelicieriformes are named for their chelicerae (their pincers) and have pedipalps and four pairs of walking legs). Myriapoda are named after their “many feet”. Unlike many of the arthropods, whose evolution has tended to fuse segments, the myriapods still have many segments. The centipede has one pair of walking legs per segment and tends to be carnivorous. The millipedes have two legs per segment and are herbivorous. Hexapoda are noted for their segmentation: head, thorax, and abdomen. They have three pairs of legs and usually two pairs of wings. In terms of segmentation, Crustacea are the middle of the spectrum between hexapod and myriapoda (Lobsters for instance, have nineteen pairs of appendages, many highly specialized). Unlike the other classes, however, crustaceans are mostly marine invertebrates.

Who rules the earth? While human impacts on the ecosystem are dramatic and increasing, it can be argued that the hands of the earth still rest in the arthropods. The arthropod phyla wins the cake in species diversity, distribution, and numbers. Unlike the other invertebrate groups, over a million species have been described.

Invertebrate Animals group four: deuterostomes

As stated in the start of this hub, animals are known for their development through embryos. Asexual reproduction, regeneration, and larval stages complicate things, but animals generally derive from embryos. One embryonic feature that has been discussed is the inward folding that develops into endoderm and mesoderm tissue. What has been glossed over is that the mesoderm does not form in the same way for all phyla, and the fate of the blastopore, (the first hole which forms during the infolding of tissue (gastrulation). In the complex bilaterans the blastopore becomes the mouth or the anus. Further development is required if a full digestive tract is to be created. The invertebrates dealt with so far have all been protostomes. In these invertebrate animals, The blastopore develops into the mouth. In deuterostomes (including echinoderms and vertebrates) the development in echinoderms and vertebrates, the blastopore forms the anus. Embryonic development is a highly conserved feature in animals, meaning that embryos change far less rapidly than adult body plans. Since embryonic development leads to such diversity, differences at basic embryonic levels are highly signficant. This is why echinoderms like the starfish have the closest common ancestor with humans than the other invertebrate groups in this hub.

Of the phyla in this hub, humans are most related to the sea stars and other echinoderms
Of the phyla in this hub, humans are most related to the sea stars and other echinoderms

Echinodermata: (starfish and its relatives)

The echinoderms are a diverse bunch of marine invertabrates but the all show a five arm body plan. This plan does not always result in a five extended arms as certain classes fold the arms together in a sort of ball (such as sea urchins and sand dollars). The unique structure of the echinoderms is the water vascular system which extends into tube feet. The tube foot consists of an ampulla that holds water inside the echinoderm and a podium that lies outside. In order to move water is pushed from the ampulla into the podium, which then pushes the water backwards into the ampulla. This backwards push deforms the podium, allowing movement. The push also creates a suction force that allows the echinoderm to stay rooted to the substrate it’s attached to. The suction force can also be maintained for some time. Unlike humans that often find it difficult to open sealed jars, the sea stars can maintain a steady force for hours, making animals such as clams favored prey.

Fun Fact: the sea cucumber, perhaps the most bizarre of all echinoderms, is not particularly mobile and has evolved the ability to eviscerate its own organs. When a predator is detected the sea cucumber literally pukes its guts up in a desperate gesture to frighten and confuse its attackers.

Final Thoughts: life without a spine is a fact for the majority of invertebrate animals on this planet. This hub is merely an introduction, as there are about 35 animal phyla (only one of which is vertebrate). The mentioned phyla are merely the best described and have the most known species. Moreover, this hub is not intended to espouse the idea that there has been a linear process from “lower” or ancestral animals towards “upper” derived animals. Evolution is not that simple and all existing species have adapted to live in today’s world however simple or complex they might be. Enjoy the following video and check out some links.

The Giant Sunflower Star

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skyfire 6 years ago

Cool hub, thanks for letting me know about this hub. :)

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