Invertebrates and Chordates
Invertebrates
Invertebrates are animals that do not have backbones. They make up two thirds of the total animal population globally. Vertebrates have characteristics like the lack of cell walls, multi-cellular bodies, the lack of backbones and inability to manufacture food on their own. The animals depend on tree leaves, algae, wood and other invertebrates. According to scientists, vertebrates are classified into groups called phyla. There are five categories of phyla, which feature 23 other smaller groups. The main invertebrate groups are phylum Cnidaria, phylum Annelida, phylum Mollusca, phylum Arthropoda and phylum Echinodermeta.
This paper will outline the main classifications that zoologists use in the grouping of invertebrates.
Phylum Cnidaria
This classification is made up of four main groups which include Anthozoa, Cubozoa, Hydrozoa and Scyphozoa. Examples of invertebrates in this phylum include sea anemones, corals, fire corals, jellyfish and hydras. They are less complex marine animals with a nervous system. The organisms that fall under this phylum Cnidaria are characterized by tentacles around their mouths, and each tentacle is covered with sting cells called nematocysts. The eyes of Cnidaria are complex, and they also have feathery hydroids. Besides, the animals live in colonies and reproduce through budding both sexually and asexually.
Phylum Annelida
This group is made up of segmented worms and has three main classes; class Polychaeta (mostly comprised of marine worms), class Hirudinea (mainly freshwater), and class Oligochaeta (predominantly terrestrial worms). Their execration is through organs called Nephridia which resemble coiled tubes. The circulatory system of annelids, especially earthworms, is through a series of blood vessels which are segmented into dorsal and ventral blood vessels and form the main blood pumping structures. Oxygen is carried by blood pigments called hemoglobin. The annelids have a nervous system which cannot be located on the ventral nerve cord. Annelids are protosomes whose body parts are segmented into repeated parts called metamerism and each segment is known as metamere.
Phylum Mollusca
Organisms in Phylum Mollusca have a body that is divided into three segments that include sensory and locomotion structures, excretory, digestive, circulatory structures and a mantle that secrets the shell; their main respiratory organ is the gill. The organisms that belong to this phylum are categorized into four classes. Class Polyplacophora is made up of aquatic animals that are defined by an oval shape and a shell of eight dorsal plates. They have a wide even foot that enables them to move from one place to another. The other class is called Gastropoda, which is made up of snails and slugs. Its inhabitants live in both freshwater and terrestrial habitats. Most of the animals that belong to this phylum retain their shell. On the other hand, the class Bivalvia consists of clams and oysters that are characterized by a shell made up of muscular foot for locomotive ability. They have siphons that extract water which is then passed through the gills for respiration. The other class in this phylum is known as Cephalopoda which comprises of animals like squid and octopus. They have visual system and tentacles. These enable them to swim very fast, and are also active predators.
Phylum Anthropoda
The invertebrate animals that fall under the phylum anthropoda have a tough exoskeleton which gives them protection. Anthropods are diversely populated in terms of geographical distribution and are categorized into three main classes. Class Insecta is comprised of insects, millipedes and centipedes. Several segments, each of which features a pair of legs, are their characteristics. The other class is known as Crustaceans and it is made up of lobsters, crabs and shrimps. Animals like spiders, scorpions and mites are categorized in the class Arachnids, and are 
distinguished by their stinging ability. The last class in this phylum is known as Trilobita, although it extinct.
Phylum Echinodermeta
Organisms that in this class can be found on the ocean bed and include starfish, sea urchins, and sand dollars. They lack heads and breed externally. These invertebrates have two cell layers, and excretory organs. They The Echinodermeta feed on fine particles found in water.
Chordates
These are animals with backbones, although some of them lack backbones but share similar characteristics. They are the most in complexity with an advanced brain that is compared to that of invertebrates. Chordates have several features that make them different from invertebrates. They have an inner skeleton with a backbone. Besides, their body is segmented into three appendages that include the head, arms and wings or fins. These appendages assist the animals in locomotion which is regulated by the spinal nerves generators with sensory feedback and super spinal coordinator. They move by swimming, walking or quadruped locomotion. All animals that are called chordates are characterized by a post anal tail. However, it is invisible in some animals like humans. The phylum chordate is grouped into sub-phylums like the Hemichordate, Urochordata, Cephalochordate and Vertebrate.
Vertebrate Chordates eat through the mouth and have a digestive system with a stomach and intestines. They have a compound nervous system that runs under the dorsal surface inside the spinal cord and a well differentiated head with a complex brain. Most of the animals in this category have two eyes and a hearing system. Their reproduction is sexually and respiration through lungs or gills. Besides, they have internal organs like the liver, heart, circulatory system, kidney and endocrine glands which are used for secretion of hormones that help in regulation of the functions of the body.
Vertebrate chordates have cartilages and bones that are specialized tissue with fibers and muscles helping the organisms to gain an upright posture. The bodies of some vertebrate chordates like mammals and birds are covered by either fur or hair. The sub-phylum vertebrate is the most common since it is made up of all the animals with a backbone like the fish, amphibians, reptiles, birds, and mammals. Chordates are generally more complex than invertebrates because of their strong muscles that enable them to move on land, and bilateral symmetry. Mammals and birds are warm-blooded, while the temperatures of reptiles, fish and amphibians are determined by the environment and they feed on either plants or animals.
However, non-vertebrate chordates have notochord but no backbone. Examples on non-vertebrate chordates include sea squirts, lancelets colonial drifting shapes, and larvaceans. A notochord is a support rod that runs on the back side of an animal just below the dorsal nerve chord. It supports the organism in the same way as the backbone to give it a bilateral symmetry. Non-vertebrates are mostly marine animals, although some hibernate on either sand or gravel. They do not portray signs of segmentation or scales. Most of them live in shallow waters with others at greater depths of the ocean. They main distinctive characteristics of these animals are their backbone which determines the way they move and even posture. Other features that also distinguish them include their feeding habits, reproduction, and respiratory methods. In conclusion, the characteristics named above represent the most common features of chordates and invertebrates.
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