On planet Earth, there are an infinite number of species of diverse animals. In order to understand them, it is worth studying the classification, which includes types, classes, and orders. Species are the last step of division, and therefore their number is so great. It is better to take a few basic types of animals, which are much smaller. To study the species included in them will be much more convenient.
Sponges
For a long time, these animals were considered plants. Science has studied their structure recently. The type of sponge includes a variety of animal species. Examples of them can be listed for a very long time. Representatives always live in an aquatic environment, but outwardly they differ in a very impressive way. Sponges may look like expressionless growths, cakes, twigs or lumps. The glass appearance of these animals looks incredibly beautiful and is a real masterpiece of nature - the so-called basket of Venus or sea orange seem openwork and translucent.
They have common features - for example, sponges almost do not move. Nevertheless, each species can get food without any problems - by passing through the body an impressive amount of water, the microorganisms from which are the diet of these bizarre creatures. But the most interesting thing is that the coincidence of the name of these animals with the name of household accessories for washing or cleaning: earlier for this purpose they used live sponges that are useful for the skin and can be very gentle to the touch.
Coelenterates
So, listing the types of animals, examples should be divided into categories according to their main types. The next is intestinal, low-organized beings, whose bodies consist of only two layers of cells. All of their species, with a few exceptions, live in the aquatic environment. For example, it is a hydroid evdendrium, an acropore coral, or a physiophore siphonophore. Each of these species has a unique appearance - some resemble small trees, while others resemble bird feathers. Their habit of colonizing and the structure of the body unites - as the name implies, the only cavity in the body is the intestine. All species can be divided into two groups: these are polyps located in a certain place, or jellyfish, which can be mobile.
Worms
When listing animal species, the list of which is very long, this type is also worth mentioning. Worms can be flat, ringed and round. All these species are united by a structure of several layers of cells - ectoderm, endoderm and mesoderm. The worms have no body cavities, the body consists of a parenchyma, which performs all the necessary functions. However, there is a difference between these species. Flatworms have a brain stem, roundworms have only a few longitudinal and periopharyngeal nerve rings, and ringworms have the abdominal nervous system. In addition, the latter have a closed circulatory system, which others do not.
Mollusks
It is worth studying not only primitive animal species. Classification includes much more developed organisms, for example, mollusks. This is the type that includes the maximum number of species. These are organisms that live in the sea (oysters, octopuses, mussels, squids), fresh water (toothless, viviparous fish, ponds) or in moist ground (snails, slugs). The sizes of mollusks of different species are very different, they can be very tiny (just a few millimeters) or reach more than twenty meters in length. Many lead a sedentary lifestyle, but some can move in a reactive way. These are squids and similar animal species. The classification of mollusks also includes such variants as cephalopods, bivalves and gastropods. Some have a shell consisting of several layers (horny and calcareous), but many have lost it during evolution. It combines all these types of animals, the names of which, by the way, include both snails and squids, the structure of the digestive system. It consists of three sections with the anterior, middle and hind intestines. Some individuals are distinguished by a muscular tongue with denticles, while others feed passively, simply filtering food out of an aqueous suspension with gills. In all species, the circulatory system is open and includes vessels and the heart with several atria and ventricle. Aquatic organisms breathe with gills, terrestrial organisms breathe with gills. The excretory system is represented by the kidneys, and the nervous system is scattered by nodes with several large ganglia.
Arthropods
Enumerating a variety of animal species, the list cannot be completed without mentioning these organisms. Arthropods are millipedes, scorpions, spiders, and crayfish. As a rule, these are two-sided symmetric animals with a body divided into segments. The body is covered with chitin cuticle, which serves as an external skeleton and protection for the body. Since various species of animals belong to this category, examples of the respiratory system can be diametrically opposite - these are both lungs and gills. All representatives have an open circulatory system. The shape of the body can be varied. As a rule, the body consists of several segments: head, chest and abdomen - all of these animal species have them. Examples, however, include some variations: in spiders, the head and chest are combined with the abdomen, and in ticks it is practically impossible to isolate segments.
Chordate
The most common and famous types of animals, photos of which everyone saw, belong to this type. It is the highest and suggests the presence of a skeletal axis, a neural tube. The type includes three main varieties of organisms: these are shells, skulls and vertebrates. The first animal species, examples of which are far less known than the second, most often live in the aquatic environment and settle in colonies. They have barrel-shaped or sack-shaped bodies, a ganglion nervous system, and underdeveloped sensory organs. Such organisms include the following species: ascidia, appendicularia, pyrosomes and others. These creatures breed in different ways, feed on algae, small animals, and detritus.
Interestingly, the adult form is simplified, but the larvae are much more active and differ in developed sense organs. Disembodied do not have a separate head, which determines their name. Gills breathe and do not have too many representatives, the most famous is the lancelet. Finally, vertebrates are the most famous and developed
group of animals. It includes all types of mammals, fish, birds, amphibians and reptiles. This type developed in prehistoric times. At the moment, a person knows about fifty thousand species of vertebrates.