Deuterostome

Deuterostome Definition

The Word Deuterostome Means -  second mouth. Deuterostome is thus a superphylum that includes groups of animals belonging to the kingdom Animalia. These groups of animals are bilaterally symmetrical and consist of a blastopore which later develops into the anus during embryonic development. The animals belonging to the phyla Echinodermata and Chordata such as starfish, sea squirts, sea urchins and lancelets are some of the common deuterostome examples. 


Therefore, deuterostomia or deuterostome can be defined as the group of animals that belongs to the phylum of Echinodermata, Chordata and Hemichordata. All these groups are classified together under deuterostomia based on their embryological development and molecular criteria. 


On this page, students can find all the necessary information on deuterostome meaning, its characteristics and classification. 

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History of Deuterostome

Initially, based on the morphological and embryological characteristics, the phyla Brachiopoda, Chaetognatha, Bryozoa and Phoronida were included under Deuterostome. But in the year 1995, this superphylum was again redefined on the basis of DNA molecular sequence analyses. In this process, the lophophorates were removed from Deuterostome and was combined with other protostome animals to form the superphylum Lophotrochozoa. The phyla Echinodermata,  Ambulacraria and Chordata are included under the refined classification of the superphylum Deuterostomia. 


The phylum Chordata again consists of two marine groups, i.e., Cephalocordata (includes the fish-like lancelets), Tunicata (for example, the sea squirts, salps and relatives) and Vertebrates (fishes, amphibians, reptiles, birds, and mammals).


Phylum Ambulacraria, on the other hand, consists of all the exclusively marine Echinoderms (for example, sea stars, brittle stars, sea urchins, sand dollars, sea cucumbers, and sea lilies) and Hemichordates (includes the soft-bodied and benthic worm-like animals).

Classification of Deuterostomia

Deuterostomia are mainly a group of animals (Kingdom: Animalia) that are characterized by their process of anus formation. In this group of animals, the development of the anus starts before the formation of their mouth during embryonic development. Some of the common Deuterostome examples include vertebrates, sea stars, and crinoids. The deuterostomes consist of three major clades which are - Chordata, Echinodermata and Hemichordata. This superphylum along with Protostomia and Xenacoelomorpha, lead to the formation of Bilateria, which includes animals having a bilateral symmetry and three germ layers.


A detailed classification of deuterostomes along with different subphylum have been listed below. One can refer to this classification to know what is a deuterostome and how it evolved.

Superphylum Deuterostomia

  1. Phylum Chordata - It includes the vertebrates, tunicates, and lancelets. They consist of two subphyla, two infraphyla and three superclasses.

  • Subphylum Cephalochordata (lancelets)

  • Subphylum Tunicata (tunicates)

  • Subphylum Vertebrata (mammals, reptiles, amphibians, birds and fish)

  • Infraphylum Agnatha (jawless fish, hagfish and lampreys)

  • Infraphylum Gnathostomata (includes jawed vertebrates such as mammals, reptiles, amphibians, birds, bony fish and cartilaginous fish)

  • Superclass incertae sedis (includes mainly the cartilaginous fish such as sharks, skates, rays, and chimaeras)

  • Superclass Osteichthyes (bony fish – ray-finned fish and lobe-finned fish)

  • Superclass Tetrapoda (includes four-limbed vertebrates such as mammals, reptiles, amphibians and birds)

  1. Phylum Hemichordata - Also known as acorn worms, hemichordates are the sister group of Echinoderms.

  2. Phylum Echinodermata - This phylum includes the group of sea stars, brittle stars, sea lilies, sea urchins and sea cucumbers. They are again categorised into three sub-phyla.

  • Subphylum Asterozoa (sea stars and brittle stars)

  • Subphylum Crinozoa (sea lilies)

  • Subphylum Echinozoa (sea urchins and sea cucumbers)

The phyla Echinodermata and Hemichordata together form the clade Ambulacraria, which is assumed to be the sister clade to Xenacoelomorpha and also form the Xenambulacraria group.

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Characteristics of Deuterostome

Some of the Characteristics of the Deuterostome is Mentioned below:

  1. Radial Cleavage of the Zygote:  The four zygotic cells undergo the process of parallel and perpendicular division to the original body axis in a symmetrical manner.

  2. Indeterminate or Regulative Cleavage: This indicates that the fate of the embryonic cells are not predetermined and can be altered during the early stage of embryogenesis if the cells are moved out to a different location. Each of the embryonic cells has the ability to develop into a complete embryo if they are isolated.

  • The coelom of a deuterostome develops from the buds of the embryonic gut. It can be defined as a fluid-filled body cavity that is lined with mesoderm. 

  • Distinctive larval forms can be found in a number of deuterostomes. 

  • During the development process, the mouth of a deuterostome starts developing from the opening into the embryonic gut instead of the blastopore, which eventually develops into the anus. 

  • The blastopore found in deuterostomes can be defined as a small indentation in the blastula, which migrates to the opposite end forming the endodermal layer.

  • Enterocoelom: In this group of animals, the coelom formation begins at the gastrula stage of development. It is formed from the fusion of internal outgrowths of the endodermal lining of the archenteron, i.e. the primitive gut. These outgrowths pinch off to form the mesodermal layer between the ectoderm and endoderm. The coelomic pouches later fuse together to form the coelomic cavity.

  • In the egg-laying deuterostomes, the ectoderm is formed by the peripheral cells of the gastrula. This is important for the development of skin, hair and nervous system. Mesoderm which is present between ectoderm and endoderm is also important for the formation of connective tissues, muscle, skeletal system, kidneys, blood and heart.

  • In mammals, the blastocyst (which is equivalent to blastula) forms the placenta. The inner cells, on the other hand, develops into three primary germinal layers.

Phylum Echinodermata

The term Echinodermata is formed by two Greek words, i.e., ‘Echinos’ meaning spiny and ‘Dermos’ meaning skin. Thus the phylum Echinodermata got its name as it includes mostly the marine creatures owing a spiny skin. The phylum includes a collection of about 7,000 living species of exclusive marine organisms. Sea stars, sea cucumbers, sea urchins, sand dollars, and brittle stars are some of the common examples of echinoderms. Whereas, no freshwater, as well as terrestrial echinoderms, are known or discovered to date.

Some of the Common Characteristics of Echinoderms are:

  • They are groups of marine animals.

  • All echinoderms have bilateral symmetry during their larval stage. 

  • The adult echinoderms have radial symmetry.

  • An interesting feature of all the echinoderm animals is their power to regenerate. They still can undergo the process of regeneration even if  75 percent of their body mass is lost.

  •  Examples: starfish, sea urchin, sea lily, sea cucumber, etc.

Morphology and Anatomy of Echinoderms

The adult echinoderms have a calcareous endoskeleton which is made up of ossicles. Although all echinoderms have a bilateral symmetry during their early larval stages, they tend to exhibit pentaradial symmetry. The epidermal cells present in the echinoderms are responsible for the development of the endoskeleton.  The pigment cells, on the other hand, gives vivid colours to these animals. Gonads are present in each arm of these animals as well as cells that are laden with toxins can also be found in the Echinoderms. 


In some of the echinoderms (for example sea stars), each arm bears two rows of tube feet on the oral side which help them to attach with the substratum. These animals also possess a true coelom which is modified into a unique circulatory system known as the water vascular system. 

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Water Vascular System

  • The organisms belonging to the phylum Echinodermata, possess a unique ambulacral or water vascular system

  • They consist of a central ring canal as well as radial canals that is extended along each arm. 

  • Water circulates with the help of these canals and thus facilitates gaseous exchange, nutrition, predation and locomotion. 

  • The water vascular system can project in the form of tube feet through the holes present in the skeleton. These tube feet can easily expand or contract on the basis of the volume of water present in the system of that arm. 

  • The echinoderms can either protrude or retract their tube feet by using hydrostatic pressure. 

  • After entering the madreporite of the echinoderm on the aboral side, the water passes into the stone canal and later moves into the ring canal. 

Nervous System

The nervous system in the echinoderms has a relatively simple structure consisting of a nerve ring at the centre and five radial nerves which extend outwards along the arms. Structures that are analogous to the brain or are derived from the fusion of ganglia can not be found in these animals.

Excretory System

Echinoderms excrete their bodily fluids through podocytes. These are cells that are specialized for the ultrafiltration of bodily fluids and are found near the centre of echinoderms. The podocytes are connected by an internal system of canals to an opening called the madreporite.

Reproduction

They are sexually dimorphic and undergo an external mode of fertilization. They tend to release their eggs and sperm cells into the water. While in some species of Echinoderms, the larvae divide asexually and multiply before reaching sexual maturity. Echinoderms may also reproduce by asexual method and can regenerate their body parts as well that are lost in trauma.

Phylum Chordata

They are characterised by their presence of notochord. There is also a presence of dorsal hollow nerve cord and pharyngeal slits in this phylum. In the case of vertebrates, these notochord is replaced by the vertebral column. Some of the common examples of Chordata includes fishes, birds, reptiles, amphibians, birds and mammals. The Phylum Chordata is classified into three subphyla, i.e., Urochordata (tunicates), Cephalochordata (lancelets) and Vertebrata (vertebrates).


Some of the common features that can be found in the Chordates show four features, at different stages in their life. They are mentioned as follows:

  1. Notochord – It is a longitudinal rod that is made up of cartilage and runs between the nerve cord and the digestive tract. The main function of a notochord is to support the nerve cord. In the case of vertebrate animals, the notochord is replaced by the vertebral column.

  2. Dorsal Nerve Cord – It is a bundle of nerve fibres that connects the brain to the muscles and other organs.

  3. Post-Anal Tail – The post-anal tail is an extension of the body beyond the anus. In some chordates, this tail consists of skeletal muscles which help in the process of locomotion.

  4. Pharyngeal Slits –They can be defined as openings that help in connecting the mouth and the throat. These openings thus allow the entry of water through the mouth, without entering into the digestive system. 

What is Protostomia?

Known as the sister clade of Deuterostome, Protostomia is a superphylum that includes a group of animals belonging to the phyla Arthropoda (e.g., insects, crabs), Molluscs (for example, clams and snails), Annelida and some other groups which have a varied digestive tract development. All these animals are classified on the basis of their embryological development and are grouped together under Protostomes. 


In Protostomia (proto meaning “first” and stoma meaning “mouth”), the mouth develops from the first opening into the embryonic gut i.e., blastopore. The coelom, also known as the body cavity, is formed due to a split in the embryonic mesoderm or the middle tissue. Additionally, the cell fates are fixed during the first cleavage of an early embryo, and therefore, the separated cells cannot form twins. They also consist of immature larval forms known as trochophores. 

Difference between Protostomia and Deuterostomia 

Both Protostomia and Deuterostomia are included under the group Bilateria, which includes all the triploblastic animals with bilateral symmetry. Bilateria comes under the subkingdom Eumetazoa which further got divided into superphylum Protostomia and Deuterostomia based on their difference in embryonic development. Deuterostome is known to be the sister clade of Protostomes. These two phyla differ in the process of coelom formation, in the cleavage of the zygote and development of blastopore.


The Major Differences Between Protostomia and Deuterostomia have Been Listed Out in the Table below:


Protostome

Deuterostome

In Protostome, the blastopore develops into a mouth and eventually forms the anus.

In the case of deuterostomic animals, the blastopore develops into the anus which later on forms the mouth.

The cleavage of a zygote in the case of protostomic animals is spiral and determinate. 

In deuterostomic animals, the cleavage of the zygote is radial and indeterminate. 

The mesoderm is formed by the migration of Mesenchyma cells.

The development of archenteron ingrowths leads to the formation of mesoderm

It is Schizocoelous, i.e., the coelom forms by splitting of solid masses of mesoderm. No archenteron is formed

It is Enterocoelous, i.e., folds of archenteron results in the formation of the coelom.

The mouth develops from the gut tunnels into the embryo forming anus.

The anus originates from gut tunnels into the embryo forming the mouth.

Their nervous system is represented by a solid, ventral nerve cord

They consist of a hollow nerve cord and pharyngeal gill slits, which makes up their nervous system.

The larval bands have multi-ciliated cells.

Their larval bands have a single cilium in each cell.

They are a group of annelids, arthropods, molluscs and flatworms.

Includes all the echinoderms and chordates

FAQs (Frequently Asked Questions)

Q1. Can Humans be Called Deuterostome?

Ans. Yes, humans can be called deuterostomes. We, the humans fall under the category of the bilateral group and are thus considered deuterostomes. By a bilateral group of organisms, we mean that the organisms are bilaterally symmetrical i.e., they have matching left and right sides to their bodies. 

Q2. Which Animals are Included Under the Superphylum Deuterostomes?

Ans. Deuterostomia, meaning “second mouth” is a group of animals that includes the phyla Echinodermata (for example, starfish and sea urchins), Chordata (for example, sea squirts, lancelets and vertebrates), Chaetognatha (includes the arrowworms) and Brachiopoda (e.g., lamp shells). All these groups are classified together under Deuterostomes on the basis of their embryological development.

Q3. Which Organ is Absent in Echinoderms?

Ans. The excretory organs are found to be absent in echinoderms. As these organisms lack specialized excretory or waste disposal organs, therefore, all of their nitrogenous waste, diffuses out through the respiratory surfaces, mostly, in the form of ammonia.

Q4. What is a Blastopore?

Ans. A blastopore can be defined as an opening through which the cavity of the gastrula (an embryonic stage in animal development) can communicate with its exterior. During the process of maturation in some animals, the blastopore evolves into the anus or the mouth. While in some other cases, it remains covered and contributes to the canal joining the primitive gut with the cavity of the neural tube.