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1. 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

2. 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.

3. 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 aredistinguished 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

4. 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

5. 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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