General characteristics of Urochordata
Sea squirts, scientifically known as tunicates, were
initially mistaken for sponges. In 1816, Lamark positioned Tunicata within the
classification system between Radiata and Vermes. Subsequently, they were
categorized under Mollusca, but in 1866, Kowalevsky reclassified them as
chordates. The defining features of tunicates as chordates are most noticeable
during their larval stages. Urochordates inhabit oceans globally, with the
majority being sedentary and some being pelagic. Their
chordate characteristics are more pronounced in the larval stage, while the
adults resemble invertebrates more closely. Therefore, their characteristics
are divided into larval and adult features.
Larval
Characteristics:
1. Notochord: The larva has a notochord in its tail, which disappears
during metamorphosis.
2. Respiration: The larval stage has gills in the pharyngeal wall for
respiration.
3. Chordate Features: Chordate characteristics are most pronounced in
the larval stage.
Adult
Characteristics:
1. Body Structure: The body varies in size and shape, is unsegmented,
and lacks a tail.
2. Body Covering: The body is covered by a test made of tunicine,
which is similar to cellulose, giving them the name "Tunicata."
3. Body Wall: The body wall consists of a single-layered epidermis,
connective tissue and muscles forming the dermis, and atrial epithelium.
4.Coelom: There is no coelom in adults.
5. Atrial Cavity: The atrial cavity surrounds the pharynx, where the
gill slits, anus, and genital ducts open. This cavity opens to the outside
through an atrial aperture.
6. Feeding: They use ciliary feeding, a common feeding method.
7. Circulatory System: They have an open circulatory system with a
ventral heart that periodically reverses its function.
8. Nervous System: The nervous system consists of a single dorsal
ganglion.
9. Excretion: Excretion is carried out by nephrocytes.
10. Reproduction:
Asexual:
Asexual reproduction occurs by budding.
Sexual: They are bisexual animals, and cross-fertilization is favored. Fertilization is external.
Understanding these
characteristics helps in studying the life cycle and classification of
tunicates in zoology.
