Pr. C. 6 ASCIDIAN(Sea Squirts)

 

Phylum: Chordata

Class: Ascidiacea

Description:                

Ascidian is a protochordate & it is commonly called as “Sea Squirts”. It is a marine, sedentary animal. It is attached to the substratum with the help of a foot. The body is covered by a thick covering tunic or test. The free end of the body is provided with two external opening called branchial aperture and atrial aperture. Mouth opens to the outside through the branchial aperture. Anuus opens to the outside by the atrial aperture. Alimentary canal is U-shaped. Pharynx contains many pairs of gill slits. All ascidians are Hermaphrodites.Fertilization is external. Development is indirect and it includes a free swimming larva called ascidian tadpole

 Tadpole contains a notochord in the tail region but the adult has no notochord. Larva undergoes retrogressive metamorphosis to become the adult

Biological Significances of Ascidians

From the evolutionary point of view, they occupy an interesting position between the invertebrate and chordate. Urochordata have affinities with Amphioxus, Balanoglossus & advanced chordates. According to Garstang’s auricularia theory, the ascidian tadpole –like larva has come from the auricularia –like larva. Ascidian tadpole larva is believed to be the direct ancestor of vertebrates

IUCN: Not evaluated