Boneless animals occupy every habitat on planet Earth, from cold Antarctica to rainy tropical forests. They are huge groups of invertebrate animals, many of them still unknown to humans, and others would even think that they are not animals because of their physical appearance.
In this article on our site called 9 animals without bones we will learn what animals without bones are called and show a list with names of animals without bones so that we know all the classifications that exist in biology.
Invertebrate Animals - Characteristics
The main characteristic that defines animals without bones or invertebrate animals is the absence of vertebrae and other bones Invertebrates do not have a internal skeleton, neither bony nor cartilaginous. Depending on the type of animal, it may have some form of support, such as the exoskeleton of arthropods.
Another important feature of this group of animals is that they are usually of a small size, except for some rare cases that we will name later ahead.
It is estimated that 95% of the animals that inhabit the Earth are invertebrate animals. These can be divided into two groups: animals with external protection and animals without protection.
Invertebrate animals with external protection are arthropods, molluscs and echinoderms, and invertebrate animals without external protection are worms, porifera, cnidarians and some others.
Phylum Porifera
Topping the list of boneless animal names we find porifera, also known as sponges. Most of them are marine (about 6,000 species), and some are freshwater (about 150 species). They abound in all seas and grow on varied substrates. Their size can vary from a few millimeters to more than two meters. They are usually irregular in shape and the color varies between species.
The phylum porifera are sessile benthic filter feeders, that is, they feed by retaining food particles that are in suspension, they settle to the bottom marine and cannot move.
Your body is made up of a system of channels through which water loaded with food and oxygen passes, and waste is also evacuated.
Reproduction in these animals can be sexual or asexual, but they are generally hermaphrodites.
Since they have no skeleton, it is the collagen fibers that form the main skeleton of sponges. They also have some spicules that can be calcareous or siliceous and are also part of their skeleton.
Phylum Placozoa
Only one known species of placozoan, Trichoplax adhaerens, found in marine waters of the Mediterranean, Atlantic and Pacific.
These boneless animals have a flattened body, 2 to 3 millimeters. They are marine benthic and move by means of flagella. They feed on the biofilm that covers the bottom surfaces. It is the free-living animal with the fewest types of cells and the least amount of DNA.
Has asexual reproduction by cleavage or budding. Eggs are formed from one of its three cell types, but no sperm or fertilization have been observed.
Phylum cnidarians
This group of invertebrate animals includes the jellyfish. There are approximately 10,000 species of cnidarians, about 20 freshwater and the rest marine.
Its body is organized in a blind sac, with a single-opening digestive cavity (mouth). Jellyfish reproduce sexually, but they can also reproduce asexually.
Coelomorph edge
The following boneless animals are divided into two groups, acoels (380 species) and nemertodermatids (9 species). The phylum acelomorphs or small worms that lack intestines are mostly marine and have a very simple internal anatomy. They are hermaphrodites, although they do not have sexual organs as such. They can also reproduce asexually.
Phylum flatworms or flatworms
There are more than 20,000 species of platyhelminthes, many of them are parasitic species of continental vertebrates, such as our dogs and cats or even us themselves, for example tapeworms.
Their digestive system ends in a blind sac, with a ventral mouth when they are free-living or forward when they are parasites. The excretory, nervous, and reproductive systems are well developed. They are hermaphrodites.
Phylum annelids
Another name for animals without bones is annelids. They are worms that are characterized by having the body divided into rings or segments. In this group we find the worms or the leeches There are about 15,000 species of annelids, many marine, some freshwater and others terrestrial.
Your body is protected by a cuticle made of collagen. Its skin is covered with cilia and various glands with chitin hairs called quetas, responsible for breathing.
Mollusk phylum
This group is made up of some 100,000 species of invertebrate animals. Most are marine, but many are also terrestrial, especially gastropods or snails They inhabit all types of environments. There are two classes, bivalves and gastropods, which have external shell and serve as protection. And one class, cephalopods, which are octopuses and squids, which have the inner shell
Phylum arthropods
Arthropods have had a great evolutionary success and are the animal group with the largest number of species, especially insects. They have a very varied size, from very small (such as Demodex spp. (0.1 mm)) to very large such as Macrocheira kaempferi up to 4 meters (less frequent).
The body of arthropods is segmented forming tagmata, thus, they are boneless animals with the body divided into head, thorax and abdomen.. They have a sclerized cuticular exoskeleton, this prevents them from growing, so they must shed every time they need to grow.
Within the arthropods we find myriapods, crustaceans, arachnids and hexapods.
Phylum echinoderms
Echinoderms are a very large and different group. There are about 7,000 species, all marine. They are dioecious animals, that is, they have separate sexes. Within this group of invertebrate animals we find crinoids, asteroids or starfish, brittle stars, sea urchins and holothurians.
They have an endoskeleton made up of plates, called ossicles or scleritesThe animal is covered with epidermal tissue, under which is the dermis and all the ossicles, which may or may not articulate with each other, depending on the species.
