Phylum Chordata, Practice Problems and FAQs

Ever tried to pass through some holes or some small areas while you were playing? If yes, then the experience will not be really good, because our body is not flexible enough to pass through some holes like an octopus does. We know that the body of gymnastics is flexible and they perform various movements. They gain this by practice.

                                     Fig: Octopus

So which is the factor that makes your body not flexible like an octopus? Our body has bones which give shape and strength to the body. Just think about your body without a backbone. Can you stand upright without a backbone? No. We used to refer to some persons as the backbone of our life, why? It means they help us to stand against any hazards in life.

If you trace the development of your backbone you will see that it all began from a flexible rod like structure. This is seen in all the embryos of higher animals. This structure that you see in the embryo is known as the notochord. But the same notochord develops to take a more rigid form in adults and changes its name from notochord to the backbone. Organisms that possess this notochord at some point in their life are considered as chordates.

Notochord is a feature that is considered as a basis for classifying animals. But let me tell you that the notochord develops into the backbone only in a few animals.

             Fig: Vertebral column

In others it may retain its original form up to the adult stage while in some it disappears in the adult stage. But whatever it is, if the notochord is present in at least one stage of the life of the animal, we call those animals as Chordates, while those that do not possess a notochord are called non-chordates. So we are going to discuss more about the chordates in this article.

Table of contents:

• Phylum Chordata
• Classification of phylum Chordata
• Differences between chordates and non-Chordates
• Practice Problems
• FAQs

Phylum Chordata

The animals with notochords can be included under the phylum Chordata. Animals which belong to the phylum Chordata have to qualify many other features along with the notochord to get included in the phylum Chordata.

Defining features of phylum Chordata

• Presence of dorsal notochord.
• Presence of dorsal hollow nerve cord.
• Presence of pharyngeal gill slits.
• Presence of post anal tail.
• Presence of a ventral chambered heart.

                     Fig: Fundamental features of chordates

Notochord

The notochord is a strong, flexible rod-like structure found in the middle of the dorsal side, between the alimentary canal and the central nervous system. Notochord is mesodermal in origin. They exist during the development of the embryo. In lower chordates, it remains throughout life, while in adults of higher chordates, it is encircled or replaced by a jointed vertebral column.

                                                                  Fig: Notochord

Dorsal hollow nervous system

The notochord is located dorsal to the central nervous system. CNS is always hollow. Neurocoel is the name of the cavity present in the central nervous system. It consists of the brain and spinal cord in higher organisms.

             Fig: Central nervous system

Pharyngeal gill slits

All chordates possess a series of paired openings called gill slits on the lateral sides at some stage in their development. In lower forms, they are maintained throughout life, while in adults of higher forms, they are lost.

                            Fig: Embryos of different vertebrates showing gill slits

Post anal tail

It is the body's elongation on the post-anal region. In many mature chordates, it could be reduced or nonexistent. The tail possesses skeletal elements and muscles. It helps in locomotion, balance, courting, and signalling.

                           Fig: Embryos of different vertebrates showing post anal tail

Ventral chambered heart

Chordates possess a chambered heart. It is located on the dorsal side. In Pisces the heart is two chambered, three chambered in amphibians and reptiles, and four chambered in Mammalia and crocodiles.

        Fig: Ventral chambered heart

Characteristics of phylum Chordata

The following are the common general characteristics of the phylum Chordata.

Habitat

Chordates are adapted to thrive in various environments which are terrestrial, aquatic (marine or freshwater) or both.

                                         Fig: Habitats of different chordates

Symmetry

Chordates are bilaterally symmetrical, i.e., their body can be divided into two equal halves through a single plane (median sagittal plane) passing through the centre of the body.

                                       Fig: Bilateral symmetry in chordates

Organ system level of organisation

Chordates have an organ system level of body organisation where division of labour can be seen between different organ systems. Examples include the digestive system, skeletal system, muscular system, respiratory system, nervous system and circulatory system.

             Fig: Organ system level of organisation in chordates (For example: In humans)

Germ layer organisation

Chordates are triploblastic. They have three germ layers which include the outer ectoderm, middle mesoderm and inner endoderm.

                    Fig: Triploblastic organisation in chordates

Body cavity

A body cavity is present between the body wall and gut wall lined by mesoderm or coelomic epithelium in chordates. This is called coelom and thus chordates are coelomates.

                            Fig: Coelom in chordates

Mode of living

Chordates can be either free-living or parasitic. Examples for free-living chordates include humans and parasitic chordates include lamprey.

Organ systems of phylum Chordata

The following are the major organ systems present in phylum Chordata.

Circulatory system of phylum Chordata

Chordates have a closed circulatory system, i.e., transport of blood occurs through blood vessels and capillaries only. They have a well developed chambered heart. Heart in chordates is present on the ventral side of the body.

             Fig: Closed circulatory system in humans

Central nervous system of phylum Chordata

There are two main parts in the central nervous system of chordates. They are the brain and spinal cord. It is single, hollow and dorsal.

 Fig: Parts of central nervous system in chordates

Classification of phylum Chordata

Phylum Chordata is divided into three subphyla. They are Urochordata or Tunicata, Cephalochordata and Vertebrata. Members of Urochordata and Cephalochordata are collectively referred to as Protochordates. Vertebrates are considered as higher chordates.

Subphylum Urochordata or Tunicata

Subphylum Urochordata is characterised by the presence of notochord only in the larval tail. It disappears in the adults. A dorsal ganglion takes the role of the nerve cord in adults, which is present in larvae. Instead of opening to the outside, the gill slits found in adults open into a cavity known as the atrium. Urochordata is also referred to as Tunicata because adult urochordates are enclosed within a tunic which is composed of a cellulose-like substance called tunicin.

                                 Fig: Internal structure of Urochordata

The larva undergoes retrogressive metamorphosis. This implies that the more-developed larva transforms into a less-developed adult. Pharynx is perforated by numerous apertures which are called stigmata. These are derived from the larval gill slits. Examples include Doliolum, Ascidia, Salpa, Herdmania etc.

                                                          Fig: Urochordates

Herdmania

It is commonly called a sea squirt. It is a filter or ciliary feeder. When disturbed, it emits jet water through both its apertures, because of it, the animal is known as the sea squirt. Its heart has no valves and blood is green due to the presence of a green pigment vanadium in the blood cells called vandocytes.

                                                 Fig: Herdmania

Subphylum Cephalochordata

Subphylum Cephalochordata is characterised by the presence of a notochord which extends from the head to the tail region. Since the notochord extends to the anterior tip of the body, the subphylum is named as Cephalochordata. Notochord in cephalochordates is present throughout life. The larvae undergo progressive metamorphosis here. This implies that the less-developed larva transforms into a better-developed adult. Pharyngeal gill slits are more numerous and better developed. Tail is present throughout their life. Examples include Branchiostoma (Sea lancelet or Amphioxus).

Branchiostoma

It is commonly called sea lancelet or amphioxus. It is called a lancelet, because it has both ends pointed like a lance. It is a marine invertebrate that lacks a backbone but has a notochord. It is used as a model organism to study how vertebrates evolved. The animal is semi-transparent and pearly white, making it possible to glimpse within organs from the outside. In contrast to vertebrates, the notochord persists until adulthood. It possesses an anteriorly slightly enlarged dorsal neural tube.

                                               Fig: Branchiostoma

Differences between Urochordata and Cephalochordata

Subphylum Vertebrata or Craniata

Vertebrates are the advanced chordates. It is characterised by the presence of a notochord only during the embryonic stage of life which later develops into a cartilaginous or bony vertebral column in the adult stage. To shield the brain, the anterior portion of the notochord is replaced with a skull or cranium. Hence the vertebrates are also known as Craniata.

          Fig: Development of notochord into vertebral column in vertebrates

Endoskeleton

The endoskeleton is composed of bone or cartilage. They have paired appendages that can be either fins, wings, paddles or limbs.

                                                 Fig: Paired appendages in vertebrates

Major organ systems

They possess a complete digestive system. Possible respiratory organs include gills, skin, buccopharyngeal cavity, and lungs. Circulatory system is of closed type. Vertebrates possess a muscular heart present on the ventral side of the body. It may have two, three or four chambers. Kidneys help in excretion and osmoregulation in vertebrates.

                                           Fig: Kidney in different vertebrates

Sense organs can be eyes, ears, tongue etc. Some vertebrates have a lateral line system present in them.

Reproduction

They lack asexual reproduction as they are higher organisms. They are unisexual and shows sexual reproduction with external or internal fertilisation.

Classification of subphylum Vertebrata

The subphylum Vertebrata is classified into two divisions. They are Agnatha and Gnathostomata. Only one class is present in the division Agnatha and that is Cyclosomata which includes animals like lampreys and hagfishes.

Division Gnathostomata consists of two superclasses, which include Pisces and Tetrapoda. Pisces contains two classes like Chondrichtyes (cartilaginous fishes) and Osteichthyes (bony fishes). But the superclass Tetrapoda contains four classes which include amphibians, reptiles, Aves and mammals.

                                           Fig: Classification of Vertebrates

Difference between chordates and non-chordates

Practice Problems

1. Which of the following is true?

1. All chordates are vertebrates
2. All vertebrates are chordates
3. All triploblastic animals are coelomates
4. All chordates have cranium (brain box)

Solution: All chordates share the notochord as a defining property. Since all vertebrates have a notochord during their embryonic stages, they are all chordates. All vertebrates share the existence of the vertebral column as a distinguishing trait. Since protochordates, urochordates, and cephalochordates lack a vertebral column or spinal column, all chordates are not vertebrates. The coelom is the area between the body wall and the mesoderm-lined gut wall. Not all triploblastic animals,those with three germ layers, including ectoderm, mesoderm, and endoderm, are coelomates (with a true coelom). Roundworms and flatworms are members of the phylum Nematoda and Platyhelminthes, respectively, and both are triploblastic. However, roundworms are pseudocoelomates (false coelom) and flatworms are acoelomates (coelom missing). The skull, which houses the brain, is present only in invertebrates and is a unique feature. Hence the correct option is b.

2. Which of the following describes a trait that all chordates share?

1. Scales
2. Jaws
3. Vertebrae
4. A dorsal hollow nerve cord

Solution: One distinguishing characteristic of a member of the Chordata is a dorsal hollow nerve cord. Some chordates, including fishes, reptiles, and birds, have scales. Scales are absent in some species, such as mammals. Cyclostomata lack jaws in their members. They are the oldest chordates and are members of the Agnatha family (jawless). Only vertebrates have vertebrae; other chordates, such as urochordates and cephalochordates lack vertebrae. Hence the correct option is d.

3. Identify the correct statement.

1. Every chordate has a cranium
2. All chordates exhibit metamerism
3. Tetrapod has two pairs of limbs
4. In chordates, the nerve cord is located on the ventral side

Solution: The term ‘metamerism’ describes the segmentation and linear repetition of various organs in the body sections. It is present in phylum Annelida. Only vertebrates, but not all chordates, have craniums. Tetrapods are a class of vertebrates that have two pairs of limbs. Even though some snakes and amphibians don't have limbs, they are nevertheless categorised as tetrapods because they descended from creatures with four limbs. Chordates have dorsal nerve cords, whereas non-chordates have ventral nerve cords. Dorsal and ventral are terms used to describe the back and front sides of an organism, respectively. Hence the correct option is b.

4. Of the following, which is a chordate?

1. Crayfish
2. Dogfish
3. Starfish
4. Silverfish

Solution: Dogfish (Scoliodon) is a chordate. It is a fish with cartilage. Phylum Arthropoda include crayfish or spiny lobster. Echinoderms include starfish. Another arthropod is the silverfish (Lepisma). Hence non-chordates include crayfish, starfish, and silverfish. So the correct option is b.

5. As opposed to non-chordates, chordates are distinguished by the _________________.

1. closed circulatory system
2. a ventral tubular nerve cord
3. cold or warm blooded
4. post-anal tail

Solution: At some point in their lives, all chordates have a post-anal tail. Tail is absent in non-chordates. Chordates have a dorsal tubular nerve cord. Dorsal and ventral are terms used to describe the back and front sides of an organism, respectively. Chordates are cold or warm blooded animals however most of the non-chordates are cold blooded. A closed circulatory system is present in all chordates. A closed circulatory system is also present in some non-chordates, such as annelids. Hence the correct option is d.

FAQs

1. Which chordata shows the bioluminescent property?
Answer: In the phylum Chordata, which also includes vertebrates, pyrosomes are tunicates which show bioluminescence. Their enormous blooms are crucial for ocean biogeochemistry and ecology. Their remarkable bioluminescence gave rise to the name Pyrosoma, which means ‘fire-body’. Energy released during a chemical reaction is the source of all bioluminescence. When compared to other sources of light, like the sun or a light bulb, where the energy is derived from heat, this is significantly different. Chemicals called luciferin and luciferase interact to produce a luminous response.

                                Fig: Pyrosome

2. What are olfactores?
Answer: The Tunicata (Urochordata) and the Vertebrata belong to the Chordata clade known as Olfactores. Since the Cephalochordata are the only chordates that are not members of this group, olfactores make up the vast majority of the phylum Chordata. This group is distinguished by a more developed olfactory system, which in the first generation of vertebrates led to the development of nostrils.

3. Which chordate class is regarded as proof that vertebrates made the transition from an aquatic environment to dry land?
Answer: In the larval stage amphibians are totally aquatic. But when they become adults, they are partially terrestrial. As a result, they are regarded as intermediary creatures in the evolution of vertebrates from aquatic to terrestrial environments.

4. What is chordate genomics?
Answer: By comparing the genomes of various animals within the chordate clade, we can study the evolution of the chordate clade. This study is known as chordate genomics. The whole DNA sequence of an organism is used in this profession as a tool. To ascertain the evolutionary history of the clade and to recreate the genome of the founding species, it compares synteny blocks (It describes the physical co-localisation of the genetic loci in the same chromosome within species or individuals), chromosome translocations, and other genomic rearrangements.

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