Diversity of Animals

 

 Diversity of Animals

The most basic type of animal is the sponge. Sponges are invertebrates and have few specialized tissues. They reproduce either asexually by budding or fragmentation or sexually, in which case they are monoecious, meaning that they are simultaneously male and female.

Next are Cnidarians. These marine organisms have radial or biradial body symmetry and have two tissue layers, the outer epidermis and the inner gastrodermis, with a jelly-like layer called the mesoglea in between.

Following cnidarians are flatworms. Flatworms are bilaterally symmetrical, meaning that the can be divided in half along a plane. (Bartleby) Their digestive systems are incomplete, and they also exhibit cephalization, meaning that their nervous systems are concentrated around their heads. This is the beginning of a brain.

More complex than flatworms are nematodes. What sets nematodes apart from less complex organisms is that they had complete digestive systems.

Next highest in complexity are arthropods. Arthropods make up about 84% of all known animal species and include lobsters, crabs, spiders, and insects. (Britannica) Arthropods have exoskeletons, a hard outer layer that provides them with protection, and segmented bodies.

Following arthropods are Mollusca, or mollusks. These include snails, octopi, and clams, and are characterized by three body parts: ventrals, muscular feet that help them move visceral mass, which houses their internal organs; and a mantle, a protective covering over the visceral mass.

Annelinda is the next most complex type of organism. Annelida are segmented worms such as earthworms that require an abundance of water to survive. They have a body plan called metamerism, meaning that they have repeating, segmented parts. At this point in the development of animals, a closed circulatory system is present.

Next are echinoderms, which include marine species like starfish, sea urchins, and sea cucumbers.  These organisms have pentaradial symmetry and can regenerate when their bodies are damaged. They reproduce sexually by releasing sex cells into the water.

More complex that echinoderms are chordates. Chordates have a notochord, a precursor to the development of a backbone. This structure is the most important feature of their nervous system. (Bartleby.) They also have three other key features: a dorsal hollow nerve cord; pharyngeal slits, which are gills; and a post-anal tail.

Following chordates are three types of fish. First are jawless fish such as hagfish and lampreys, which are the first true vertebrates. Next are jawed fish, which give life birth, which sets them apart from earlier organisms. Bony fish have an ossified calcium skeleton.

Amphibians follow fish and are characterized by their moist, permeable skin. They are tetrapods, meaning that they walk on four legs. They breathe cutaneously, meaning that they breathe through their skin.

Next in complexity are reptiles. Reptiles are amniotes, meaning that their eggs are protected by a membrane that allows them to be laid on land. Like amphibians, they are tetrapods, though they are less water dependent than amphibians. Reptiles are ectotherms, meaning that they cannot regulate their own body temperature.

Closely related to reptiles are birds. They are covered in feathers, which are modified reptile scales. Unlike reptiles, they are endotherms and can regulate their body temperature. In order to help them fly, birds have hollow bones and a system of air sacs in their bodies.

After birds are mammals, which are distinguished by their hair and mammary glands, which produce milk. Similar to birds, they are endothermic. Mammals have secretory glands and heterodont teeth. The most complex type of mammals of primates, which are set apart by their thumbs and depth perception.

 

References

Animal diversity. Britannica. https://www.britannica.com/animal/animal/Animal-diversity.

Animal Diversity. Bartleby. https://www.bartleby.com/subject/science/biology/concepts/animal-diversity

arthropod. Britannica. https://www.britannica.com/animal/arthropod