UROCHORDATES

UROCHORDATES :-

    - These are those chordates which have notochord only in larval stage not in adult one
- This phylum of chordrates also called Tunicata , which is coined by lamarck
- This urochordates are consisder in group Protochordates , which literally means old type of chordates
- In this urochordates there is about 2,000 species world wide
- they have primitive type of chordates so have great evolutionary importance to determine the changes occur during evolution
- Here are some character by which urochordates are determined are :-
* They are exclusively marine
* May be sedentary or pelagic
* Color varies grey to green
* They lack appendages
* Body consist epidermis, mesenchyme , atrial epithelium ( outer to inner)
* Notocord only in tail region in larva only
* Alimentary canal complete
* Excretory structure may be are nephrocytes, pyloric gland and neural gland
* Animal may be monoecious or bisexual
* They mainly are cold blooded animal
* cleavage is holoblastic and indirect development

@ Resemblance with vertebrate :-

- Dorsal nerve cord
- having gill slits
- Post anal tail
- absence of metamerism
- having Retrogressive metamorphism , which result lost of chordates character during development

@ Classification :-

      Here urochordates are divided into three main classes are as

1.) Ascidiacea :-

- colonial, sedentary, bottom living, tunicates having thick opaque permanent test , numerous gill slits, well developed atrium and reproduction by budding process
- this animal have 2 order are

i) Enterogona :-

         * these animal gonads are single which are present in intestinal loop
* Their neural gland present ventral to nerve ganglia
* Having cerebral eye
* larva called tadpole larva
* Ex:- Ciona (tube sea squid)

ii) Pleurogona :-
* These animal have paired gonads in lateral atrial wall
* Their neural gland may be at dorsal or ventral side of nerve ganglia
* Ex:- Botryllus (sea potato)

2.) Thaliacea :-

          these are those urochordates whose have characterized by mantle muscle , these animal are free swimming, barrel shaped, lost notochord during metamorphosis, and reproduction occured by budding . this animal divide into three order

i) Doliolida :-

* Barrel shaped , solitary animal
* Having 8 complete ring of mantle muscle
* Gill slits is very few
* Ex :- Doliolum

ii) Pyrosomida :-

- these animal are cylindrical or colonial
- Their muscle are poorely development
- There tailed larva is absent
- Ex:- Pyrosoma

iii) salpida :-

* these animal are solitary animal
* their ventral muscle are ventrally incomplete
* One pair of elongated lateral gill slits
* Ex:- salpa

3.) Larvacea :-

       these animal are most advance animal in urochordates , these have divided into 2 class

i) Endostylophora :-

- these are those animal who have bilateral symmetry
- These animal have well developed inhaled  and exhaled aperture
- Ex:- Oikopleura

ii) Polystylophora :-

* These are those animal are biradially symmetry
* These animal having only one aperture
* Ex :- Kowalewkia