Subphylum – Urochordata
Lamark placed Tunicata in between radiate and Vermes in his system of classification. Later they were kept in Mollusca. Kowalevsky kept them in chordates in 1866.
The Urochordates are also known as tunicates. These are commonly known as “sea squirts”. The 1,300 species of urochordates are known.
General Characters of Urochordates:
They possess four characteristic anatomical structures as embryos: a flexible body-length rod notochord that provides resistance against muscular contractions and allows for more efficient movement; a dorsal, hollow, nerve cord that forms the central nervous system; slits at the beginning of the digestive tract (the pharynx) that allow filter-feeding and gas exchange; and a post-anal tail.
They show features of chordates clearly in the larvae stage, while adults show more like invertebrates than chordates.
- Body shows variation in size and form.
- Unsegmented body, absence of a tail
- The body is covered by a test. It is formed by tunicine which is rallied to cellulose. Hence the name Tunicata
- Body wall shows one layered epidermis, the dermis is made by connective tissue and muscles, and atrial epithelium.
- Celome is absent.
- The atrial cavity surrounds the pharynx, into this cavity the gill slits, anus, and genital ducts will Open. It opens through the atrial aperture.
- Ciliary mode of feeding is common
- The respiratory system contains gills in the pharyngeal wall.
- Ventral heart
- The nervous system is represented by a single dorsal ganglion in the adult
- Excretion by nephrocytes
- Asexual reproduction by budding.
- Bisexual animals, Cross-fertilization.
- External fertilization.
Subphylum Urochordates divided into three classes:
- Class- Thaliacea
- Class- Larvacea (appendicularia)
- These are sedentary tunicates.
- The body is covered by a test.
- The pharynx is large and contains gill-slits.
- Notochord, nerve cord, and tail are absent in adults.
- These are bisexual animals.
- Life history includes a typical tadpole larva.
The class Ascidiacea is divided into two orders.
Order 1. Enterogona
- These ascidians bear one gonad in the intestinal loop. Neural gland is ventral to the ganglion. Tadpole larva is seen.
- Ex: Ascidia and Ciona
Order: 2. Pleurogona
In these ascidians, gonads are paired and are present in the atrial wall. Neural gland is dorsal to the ganglion:
Ex: Herdmania, Botryllus.
- These urochordates are free-swimming and pelagic forms.
- They are covered by the transparent test.
- The brachial and atrial apertures are placed at the anterior and posterior ends.
- The pharynx is small.
- Notochord, nerve cord, and tail are absent in the adult.
- Asexual reproduction is by budding.
- These are bisexual animals.
- Tailed larva may be present or absent.
- Alternation of generations can be seen in the life history.
The class Thaliacea is divided into three orders:
- Doliolida (Cyclomyarla) : Ex: Doliolum
- Pyrosomlda: Ex: Luminescent colonial form.
- Salpida (Hemimyaria): Ex: Salpa
Class – Larvacea (appendicularia):
- These are free-swimming pelagic tunicates.
- True test covering is lacking.
- They show loose gelatinous house.
- This house is useful for filter feeding.
- Two-gill slits are present
- Atrium absent
- Notochord and nerve cord are persistent
- They show tail throughout their life.
- Neotenic forms are included.
- Ex: Oikopleura.
You Can also read: Characteristics and Classification of Phylum- Chordata
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