Urochordata: Definition, Meaning, Classification, characteristics, Examples

What Are Urochordata?

Urochordata or Tunicata is a subphylum of chordates and these are animals which develop a notochord at some point in their life cycle. This subphylum has characteristics like, dorsal hollow nerve cords, pharyngeal slits and posterior tail, species in this subphylum include vertebrates and cephalochordates and urochordates.

Urochordates, more specifically, are members of this phylum that are mostly invertebrates and these chordate characteristics are mostly evident during the larval stage. In their adult form, they become sessile and live encased in a tunic which is made up of a cellulose-like material. This subphylum comprises fascinating animals like sea squirts, salps, and larvaceans that paint a very rich picture of the versatility and richness of Chordata.

Taxonomy & Classification

The classification is described below-

Classes of Urochordata

The subphylum includes three primary classes:

Ascidiacea (Sea Squirts): Laid on the sea bed as adults with cylindrical bodies protected by a tunic, with two openings, incurrent and excurrent siphons through which the organism feeds on particulate matter. Adults are bipedal with lungs. However, larvae are free-swimming and shed their notochord and tail during metamorphosis.

Thaliacea (Salps): Salps are floating animals, long, barrel-shaped, and gelatinous. They can be colonial also. It presents itself in a life cycle with both single-celled and colonial forms that allow for both sexual and asexual means of reproducing while at the same time filtering plankton with their bodies.

Larvacea (Appendicularia): Larvaceans are small planktonic animals, which never lose their notochord or tail and build mucous houses to filter food. They look like tadpoles to some extent, particularly to retain the basic forms in the freeloader behavioural mode.

General Characteristics of Urochordata

The characteristics are explained-

Body Structure

• Urochordates are enclosed in a kind of cuticle called the tunic, which is made up of tunicin which is similar to cellulose.

• This tunic offers both physical protection and rigidity from predators to the wearer.

• In regards to their physical anatomy, urochordates show dramatic metamorphosis from their larval stage to their adult stage.

• During the larval stage, the organisms are motile and possess a notochord, a dorsal nerve cord, and a tail, during the adult stage however many of these species are sessile, especially sea squirts and have very few structures.

• Chordates possess a notochord which exists in the larva stage and helps in giving support and movement.

• It is usually dissolved during the process of metamorphosis to adapt to the adult form of the organisms.

Life Cycle

• Urochordates in their life cycle pass through a motile larva phase to a generally sessile adult phase which may be much more developed.

• The development includes the involution of the notochord and tail in many species and the evolution from swimming or crawling to a sedentary or floating existence.

• The adult life stage of urochordates, the class, which comprises adults that gave rise to tunicates, is quite diverse.

• Members of Ascidiacea are mostly sessile and normally attach themselves to a substrate/substratum while those of Thaliacea and Larvacea are more or less free-moving, floating or swimming in the midwater or epipelagic zone of the ocean.

Anatomy & Physiology

The anatomy and physiology are described-

External Features

• The tunic is a strong, outer layer made up of a cellulose-like protein called tunicin, formed as an extra layer of protection.

• It gives the more or less the skeletal frame and protection from enemies, covering the whole body.

• The incurrent siphon takes in water into the body for food and oxygen, on the other hand, the excurrent siphon ejects the food-filtered water and waste products.

Digestive System

• The digestion process includes a mouth, pharynx, oesophagus, stomach, intestine, and anus.

• Cereals are stuck in the pharynx and enclosed in mucus to the stomach which breaks down the food particles.

Circulatory System

• An open circulatory system present characterised by the presence of two siphons; incurrent (oral) and excurrent (atrial) siphons.

• The blood circulates through vessels that enter into body cavities so that they are in contact with the tissue.

Nervous System

• The nervous system is quite small and delimited by a cerebral ganglion situated just behind the incurrent siphon and nerve cords that spread throughout the organism.

Reproductive System

• The majority of urochordates represent hermaphrodites, they are organisms which have both male and female reproductive habits.

• They can reproduce sexually by shedding eggs and sperm into water and the process of external fertilization.

Respiration & Feeding

The respiration and feeding process is described below-

Filter feeding mechanism

• Urochordates are filter-feeding animals.

• Water that contains the food is ingested through the incurrent siphon and is lined with a mucous-pharyngeal basket that filters out the food particles.

• They move these particles through cilia and into the digestive system.

Pharyngeal Slits

• The pharyngeal slits which are situated in the pharyngeal basket act as an apparatus for feeding as well as respiration.

• Because of the slits that are present in the surrounding walls of the tubes, water is allowed to flow with diminished oxygen levels while at the same time releasing carbon dioxide in the process of gas exchange.

Habitat & Ecology

The habitat and distribution are explained:

• Urochordates are strictly aquatic dwelling animals that are distributed within different marine habitats such as the coastal area, abyssal zone, and pelagic zone.

• Ascidiacea is a sessile lifestyle living in intertidal and subtidal environments while Thaliacea and Larvacea float in the water column at various depths.

• Urochordates filter parasites and debris since the organisms are filter feeders and this expelled water is clean.

• They are key elements of the marine ecosystem as they are the food to many species and participate in nutrient recycling and energy transfers, which in turn support marine species’ survival and the overall health of the ocean ecosystems.

Reproduction & Development

The reproduction and development are listed below-

Asexual Reproduction

Budding in colonial forms

• Most of the colonial urochordates especially in Ascidiacea have a feature of asexual reproduction which involves budding.

• This process entails reproduction that is asexual where several individuals are produced from the body of a single organism, essentials for the colony’s growth and expansion.

Sexual Reproduction

Hermaphroditic nature

• In the majority of urochordates, such animals are hermaphroditic, this means that they contain elements of both the male and female reproductive systems.

• This makes them able to lay eggs as well as develop a sperm chamber.

Fertilization process

• In urochordates, there is internal and external fertilization.

• Frequently, male and female gametes are poured into the water where the external development of gametes takes place.

• In others, it is internal within the body cavity, specifically, the process of fertilization.

Larvae

• Larval form is very important for urochordates.

• The larvae are mobile with a notochord and tail and with a dorsal nerve cord.

• These features are important for movement and attachment to the proper surface.

• At the time of settlement, most of the larvae undergo a direct metamorphosis in which all of the tail and notochord are resorbed and a sessile or a pelagic adult is constructed depending on the class.

Urochordata NEET MCQs

Q1. Excretion in urochordate occurs through

1. Glomerulus

2. Nephrocytes

3. Neural glands

4. 2 and 3

Correct answer: 4) 2 and 3

Explanation:

Nephrocytes: These are specialized cells found throughout the tunicate's tissues that accumulate waste products. They potentially contribute to detoxification by concentrating and removing wastes from the organism's system.

Neural gland: the neural gland can be likened to a rudimentary kidney. It is responsible for the filtration of bodily fluids, thereby purifying them of metabolic wastes. These waste substances are effectively expelled into the pharynx, facilitating their subsequent removal from the organism as they are carried out with the flow of water. This mechanism is crucial for maintaining internal homeostasis and preventing the accumulation of potentially harmful substances within the tunicate's body.

Hence, the correct answer is option 4) 2 and 3.

Q2. Notochord, which is a characteristic feature of chordates, is present only in the larval tail in

1. Herdmania

2. Salpa

3. Amphioxus

4. Both 1 and 2

Correct answer: 4) Both 1 and 2

Explanation:

Herdmania, a tunicate, and Salpa, another tunicate, are the only species that have the notochord, a chordate trait, in their larval tail.

But in Amphioxus (a lancelet), the notochord is not limited to the larval tail; it runs the length of the body.

Hence, the correct answer is option 4) Both1 and 2.

Q3. Retrogressive Metamorphosis is in

1. Cephalochordates

2. Urochordates

3. Agnatha

4. Gnathostomata

Correct answer: 2) Urochordates

Explanation:

Regression metamorphosis is essentially the degenerative changes in which active larvae morph into sessile adults. Only in the larvae of the Urochordata subphylum is the notochord detected, specifically in their tail. On becoming adults, these organisms depict reduced tails as well as less complex nervous systems because they don't have very active lifestyles due to their settled nature. For example, commonly found Urochordates are Ascidia, Salpa, as well as Doliolum, while Branchiostoma forms a cephalochordate and Balanoglossus is a part of hemichordates.

Hence the correct answer is Option (2) Urochordates.

Also Read:

• MCQs on Characteristics Features of Urochordata

• Phylum Platyhelminthes

• Digestive system of Earthworms

FAQs on Urochordata

What are Urochordata and why are they also called Tunicates?

Urochordata, also called Tunicata, is a subphylum of Chordata. They show characteristic chordate features like notochord, dorsal nerve cord, and pharyngeal gill slits only in their larval stage. The notochord is restricted to the tail region, hence the name uro (tail) chordata. Adults lose these features and become sessile. They are called Tunicates because their body is covered with a tough outer cover made of tunicin, a cellulose-like substance. This tunic acts as a protective covering and is unique among animals.

What is the general body structure of Urochordates?

The body of Urochordates is enclosed in a tunic. The larval form is free-swimming, with a notochord and nerve cord. Adults usually lose the notochord and nerve cord, becoming simple, sac-like, and mostly sessile. They possess two siphons, an incurrent siphon for entry of water and food, and an excurrent siphon for water exit. The pharyngeal basket with slits helps in both feeding and respiration. This structural adaptation helps them survive in marine environments.

What are the main classes of Urochordata?

Thus, classification is based on habitat, structure, and retention of chordate features. Urochordata is divided into three classes:

1. Ascidiacea (Sea squirts): Sessile adults with two siphons and a sac-like body.

2. Thaliacea (Salps): Free-floating, barrel-shaped forms that often occur in colonies and move by jet propulsion.

3. Larvacea (Appendicularia): Retain larval features throughout life (neoteny) and secrete mucous “houses” to trap food particles.

How do Urochordates perform respiration and feeding?

Urochordates are filter feeders. Water enters through the incurrent siphon and passes into the pharyngeal basket, which has numerous gill slits lined with mucus. Food particles get trapped in mucus secreted by the endostyle, and cilia transport them to the digestive tract. The excess water exits through the excurrent siphon. Gas exchange also occurs across the pharyngeal wall, where oxygen is absorbed and carbon dioxide is released. Thus, feeding and respiration are linked in these organisms.

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