General characters and classifications of protochordata
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Answer:
The organisms belonging to the Protochordata are generally known as the lower chordates. They don’t form a “proper” taxonomic group and are only classified as such for convenience purposes. However, they do form a major division of Chordata. They are also known as Acraniata because they lack a true skull. They are divided into three sub-phyla- Hemichordata, Urochordata, Cephalochordata.
Characteristics of Protochordata
They are generally found in marine water.
Their body is bilaterally symmetrical, triploblastic, and coelomated.
At a certain stage of their lives, their body develops a long, rod-like structure for support called notochord.
They exhibit organ system level of organization.
Eg., Herdmania, Amphioxus.
Classifications of Protochordata
Hemichordata
They are found in marine water.
Some live solitarily, and some stay in colonies.
The body is cylindrical, unsegmented, and stout.
The body is divided into proboscis, collar, and trunk.
The collar bears arms and tentacles.
They have a complete digestive system.
They respire through gills or general body surface.
The circulatory system comprises of a heart with two longitudinal vessels.
The blood has no colour and corpuscles.
The proboscis gland or glomerulus make up the excretory system.
Sexes may be separate or united and fertilization is either internal or external.
Eg., Cephalodiscus, Rhabdopeura
Urochordata or Tunicata
They are found in the marine environment.
They are sessile and filter-feeders.
They are also known as tunicates because their body is surrounded by a leathery sheath composed of tunicin (cellulose).
The notochord appears in the larval stage in the tail of the larva and disappears in the adult. This is known as retrogressive metamorphosis.
The neural tube in the larva is replaced by a dorsal ganglion in the adults.
Respiration occurs through gills.
They have an open circulatory system.
The excretory organs are absent.
They reproduce asexually by budding.
Eg., Herdmania, Selpa
Cephalochordata
They are marine and filter-feeders.
The notochords remain throughout life and extend up to the head region.
The nerve cord and the tail also remain throughout life.
Solenocyts are the excretory organs.
They respire through gills which open in the atrium.
The body wall comprises of myotomes.
Eg., Amphioxus