Cephalochordata vs. Urochordata

Attribute	Cephalochordata	Urochordata
Phylum	Cephalochordata	Urochordata
Body Symmetry	Bilateral	Bilateral
Body Plan	Long, slender, fish-like	Varies (sessile or free-swimming)
Notochord	Persistent throughout life	Present only in larval stage
Endostyle	Present	Present
Pharyngeal Slits	Multiple, used for filter feeding	Present, used for filter feeding
Heart	Simple tubular heart	Simple tubular heart
Respiration	Gills	Gills
Reproduction	Sexual	Sexual
Development	Direct	Indirect (larval stage)

Introduction

Cephalochordata and Urochordata are two subphyla of the phylum Chordata, which includes all animals possessing a notochord at some stage of their development. While both subphyla share certain characteristics, they also exhibit distinct attributes that set them apart. In this article, we will explore the key features of Cephalochordata and Urochordata, highlighting their similarities and differences.

Anatomical Features

Cephalochordates, commonly known as lancelets or amphioxus, are small, elongated marine animals that retain their chordate characteristics throughout their entire life cycle. They possess a notochord, a flexible rod-like structure that provides support and acts as a precursor to the vertebral column found in higher vertebrates. The notochord extends from the anterior to the posterior end of the body, providing stability and allowing for efficient locomotion.

Urochordates, on the other hand, also known as tunicates or sea squirts, exhibit a unique feature called the tunic. The tunic is a tough, non-living outer covering made of cellulose-like material that surrounds the soft body of the organism. This protective structure gives urochordates their characteristic barrel-like appearance. Unlike cephalochordates, urochordates possess a notochord only during their larval stage, which is eventually lost during metamorphosis.

Feeding and Digestive System

Both cephalochordates and urochordates are filter feeders, meaning they obtain their nutrition by filtering small particles from the water. Cephalochordates use a unique feeding mechanism called the buccal cirri, which are finger-like projections surrounding the mouth. These cirri create water currents that bring in food particles, which are then trapped in a mucus net and transported to the mouth for ingestion.

Urochordates, on the other hand, possess a specialized filtering structure known as the branchial basket. The branchial basket consists of a series of gill slits that allow water to enter the body and pass through a mesh-like structure, trapping food particles in the process. The filtered water is then expelled through an excurrent siphon. This feeding mechanism is highly efficient and allows urochordates to extract nutrients from the surrounding water.

Reproductive Strategies

Cephalochordates and urochordates employ different reproductive strategies to ensure the continuation of their species. Cephalochordates are hermaphroditic, meaning they possess both male and female reproductive organs. However, self-fertilization is rare, and cross-fertilization between individuals is the most common method of reproduction. During spawning, eggs and sperm are released into the water, where fertilization occurs externally.

Urochordates, on the other hand, exhibit a variety of reproductive strategies. Some species are hermaphroditic, while others have separate sexes. Fertilization can occur internally or externally, depending on the species. In some urochordates, such as the sea squirts, asexual reproduction through budding is also observed. This process involves the development of a bud on the adult organism, which eventually detaches and grows into a new individual.

Nervous System and Sensory Organs

The nervous system of cephalochordates is relatively simple compared to higher vertebrates. They possess a dorsal nerve cord that runs along the length of the body, located above the notochord. This nerve cord gives rise to various nerves that innervate different parts of the body. Cephalochordates also have a specialized sensory structure called the amphioxus eye, which is sensitive to light and helps in detecting changes in the environment.

Urochordates, on the other hand, have a more complex nervous system compared to cephalochordates. They possess a ganglion, or a concentration of nerve cells, located near the pharynx. From this ganglion, nerves extend throughout the body, allowing for coordination and response to stimuli. Urochordates also possess sensory organs called ocelli, which are light-sensitive structures that help in perceiving changes in light intensity.

Evolutionary Significance

Both cephalochordates and urochordates hold significant evolutionary importance as they provide insights into the early stages of chordate evolution. Cephalochordates, with their retention of chordate characteristics throughout their life cycle, are considered to be the closest living relatives of the vertebrates. They offer valuable information about the ancestral features that eventually led to the development of vertebrates.

Urochordates, although they undergo significant morphological changes during their life cycle, still retain certain chordate characteristics during their larval stage. This larval form, known as the tadpole larva, possesses a notochord, a dorsal nerve cord, and gill slits, resembling the features seen in cephalochordates and early vertebrates. The study of urochordates helps in understanding the evolutionary transitions that occurred during the development of vertebrates.

Conclusion

In conclusion, while both Cephalochordata and Urochordata belong to the phylum Chordata and share certain chordate characteristics, they exhibit distinct attributes that differentiate them from each other. Cephalochordates, such as lancelets, retain their notochord throughout their life cycle and possess unique feeding mechanisms and sensory structures. Urochordates, on the other hand, undergo metamorphosis and possess a tunic, a specialized filtering structure, and various reproductive strategies. The study of these two subphyla provides valuable insights into the evolutionary history and development of chordates and vertebrates as a whole.