Cephalochordata: Definition, Meaning, Classification, characteristics, Structure, Function

What Are Cephalochordata?

Cephalochordata is a subphylum that belongs to the phylum Chordata and refers to small, marine invertebrates collectively called lancet or amphioxus. These animals are long and thin, somewhat similar to fish, and are transparent, also they have notochords, nerve cords, and pharyngeal slits in the entire life cycle. Cephalochordate occupies an important place in animal classification because it has sensed the evolution of vertebrates.

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This Story also Contains

1. What Are Cephalochordata?
2. Taxonomy and Classification Of Cephalochordata
3. Morphological Characteristics
4. Habitat And Distribution Of Cephalochordata
5. Life Cycle And Reproduction
6. Feeding And Nutrition
7. Recommended video for "Cephalochordata"

They are basal to the majority of chordates therefore they prove useful in giving insights into the early adaptations that were probably present in chordates before the vertebrates evolved. Research on Cephalochordata is important in providing information on the biology of chordates and in the larger scheme of things, the animal kingdom.

Taxonomy and Classification Of Cephalochordata

The classification is described below-

Phylum Chordata

Animals included under Phylum Chordata include animals that possess at some stage a notochord, dorsal nerve cord, pharyngeal slits as well as a post-anal tail. It includes vertebrates and two invertebrate subphyla: The following are classifications of the animals; Urochordata, also known as tunicates and Cephalochordata, also known as lancelets based on these characteristics and phylogeny.

Subphylum Cephalochordata

Cephalochordata which are also known as the lancelets are small marine chordates that resemble fish, translucent in body, with notochord running from head to tail, dorsal nerve cord and pharyngeal slits used for filter feeding. This extant reptile is kol Cuisine found throughout shallow marine waters all over the world and is critical for the study of vertebrates.

Classification of Cephalochordata

Some families of Cephalochordata are Branchiostomatidae, which the majority of lancelets are included in, and Asymmetronidae, for example, Asymmetron. Some of the major genera comprise Branchiostoma and Epigonichthys. It also helps in analysing the levels of diversity, evolution and roles of lancelets in different marine habitats.

Morphological Characteristics

The morphological characteristics are listed below-

External Morphology

Body shape and size

Cephalochordates are also called lancelets and they have fish-like bodies that are prolate and thin in shape. These are from three centimetres up to approximately 10 centimetres in length concerning various species.

Distinguishing features

Lancelets have a weak external skeleton and mostly translucid bodies and they have rather well-distinguished organs. They also have a long notochord which runs through the length of the body a dorsal nerve cord above the notochord and several pharyngeal slits on the lateral aspect of the pharynx for filter feeding.

Internal Morphology

Nervous system

The extent of the nervous system in lancelets is modest; it only possesses a dorsal nerve cord situated along the notochord at the lateral sides of the length of the body. Ganglia or a group of nerve cells are located along the length of the nerve cord to regulate the sensory and motor points.

Muscular system

Despite the represented simple body plan, lancelets have well-developed segmented body musculature needed for their mobility and movement in the water column. Muscles are group arranged in segments, which gives one certain flexibility of movements of the body.

Digestive system

Some aspects of the organisational plan of lancelets: are feeding organs – mouth, positioned at the anterior end; the pharynx provided with numerous openings for filter feeding. Solid food is entangled by the mucous nets or those parts lined with cilia within the pharynx before they are channelled to the intestines, where they are digested and absorbed.

Circulatory system

Lancelets do have well-developed circulatory systems, open blood, ventral heart circulation and tubes that deliver blood to some parts of the body. It also provides for the distribution of oxygen and other nutrients to the tissues and organs of the body.

Respiratory system

Gill filtration in lancelets is through the body surface and the pharyngeal openings. The water is sucked into the pharynx through ciliary motion while oxygen is taken into the bloodstream, carbonases, and other wastes are let out.

Excretory system

Specialised excretory structures such as kidneys are absent in lancelets. Thus, metabolic wastes are chiefly disposed of comparatively across the body surface and perhaps by the gills or branchial openings.

Habitat And Distribution Of Cephalochordata

The distribution is described below-

Geographical Range

All the existing species are in the superclass Cephalochordata also known as lancelets and are commonly found in temperate and tropical seas covering the entire world. They are most diverse adjacent to continents and the continental shelf, but may also be found in the open ocean.

Environmental Preferences

Marine habitats

Lancelets are marine only, and they are found on sandy or muddy substrata in shallow coastal waters. They select waters with medium currents, as in these conditions, they can efficiently sift feed on phytoplankton and other organic matter in the water stream.

Specific ecological niches

Thus, the ecological role of lancelets can be evaluated within marine environments, taking into consideration their feeding habits and preferred places of existence. They are mostly reported to inhabit the sediments, albeit partially, with their head exposed for filter feeding. This adaptation helps them in feeding since they can catch the food particles while avoiding being eaten.

Life Cycle And Reproduction

The life cycle and reproduction are listed below-

Developmental Stages

From egg to adult

The life cycle of cephalochordates starts with the fertilization of eggs released in the water column. In some animals, embryos develop outside the body and undergo cleavage as well as gastrulation. They complete their development as free-swimming larvae or tornaria larvae which bear certain chordate features such as the notochord, dorsal nerve cord and post-anal tail. The tornaria larvae of lancelets go through a process of development where they finally settle at the sea floor and become mature lancelets.

Metamorphosis

Lancelets’ development entails the larval stage, which is called tornaria that forms sedentary or partially buried adults. In the course of this development, the larval features are transformed into adult features like the fully formed and efficient digestive systems, and the absorption of the tail.

Reproductive Strategies

Sexual reproduction

Cephalochordates are gonochoristic but some are simultaneous hermaphrodites that is they have both male and female gonads. They ovulate and mature sperm in their testes and thereafter, the eggs and sperm are discharged into the water body during the period of spawning. With this strategy, plants and hence sperm have higher chances of fertilizing the eggs in an aquatic environment which has a lot of water currents.

Fertilization methods

The process of reproduction in lancelets is considered an external type of fertilization, which means that eggs and sperm are released in the water at the same time. Sperm cells move to get to the eggs and impregnate them so that the embryonic process is started. Through this method, production of large numbers of offspring can be produced thus increasing reproductive capacity in different aquatic regions.

Feeding And Nutrition

The feeding and nutrition process is described below-

Diet

Cephalochordates or lancets are animals with pharyngeal slits and gills, but no thyroid gland; primary consumption is plankton and other organic debris floating in the water. These include phyto and zooplankton, detrital materials, and any other small animals they feed on from water mass.

Feeding Mechanism

Filter feeding mechanism is common in lancelets where the organisms move their heads back and forth and let the food particles stick to their mucus-coated tentacles. They possess a unique feeding structure made up of mucous nets and cilia-lined machinery well located in their pharynx. Through the process of ciliation water is pulled into the pharynx and food particles are trapped in mucous nets or on cilia surfaces. I further said that the water that passes through the gills is filtered is then ejected through pharyngeal slits.

Digestive Process

Any food particles that may fall in the phintoynx are utilised in the intestine for digestion and absorption. Lancelets have a very primitive digestive system with a mouth on the ventral surface of the animal’s head, a pharynx with gill slits for filtration and an intestine with a part for digestion. Digestive enzymes secrete compounds that allow a small portion of the food particles to be absorbed into the blood through the intestinal wall. Wastes are released through the rectum of the body via the anus.

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