Protochordates - phylogeny of protochordata Theory

Protochordates - phylogeny of protochordata Theory 

General features and phylogeny of protochordata

Near about 5 to 100 million species of organisms living on Earth today. Evidence from morphological, biochemical, and gene sequence data ggests that all organisms on Earth are genetically related. It is considered that all organisms descend from a common ancestor. During evolution new organisms develop when existing populations split into two groups. In such organism , lineages experience changes in characteristics occurs over time and new organism added into the ecosystem. The relationships among organisms are determined by shared characteristics, as indicated from end to end genetic and anatomical comparisons.

“Phylogeny is the study of the evolutionary development of groups of organisms.” Phylogeny is  based on the idea that all life is resultant from a common ancestor. A phylogeny is represented in a diagram known as a phylogenetic tree. The branches of the tree represent ancestral and/or descendant lineages.

Phylogenetic Tree: 

A phylogenetic tree, or cladogram, is a schematic illustration used as a visual diagram of projected evolutionary relationships among taxa. Thus Tree of Life represents the phylogeny of organism.

Phylogeny of Protochordata

Phylogenetic tree

The deuterostomes are a monophyletic group of multi-cellular animals that include the Chordata, a phylum having a unique body plan within the metazoans. Deuterostomes characteristically contained three phyla, Echinodermata, Hemichordata, and Chordata. Protochordata describes two invertebrate chordate subphyla, the Tunicata (Urochordata) and the Cephalochordata.

The notochord is only present in the tail of the larva and disappears in the adult

    Protochordata  (Acraniata)

                        

Sub-phylum

           Urochordata                                                   Cephalochordata             

        (Eg. Herdmania,)                                                (Eg. Amphioxus)                            

Protochordates are commonly called lower chordates. These animals lack  a head and a skull or cranium, so they are also known as Acraniata.  

On the property of notochord Protochordates  consists of three sub-phyla which are as follows.

1.  .   Urochordata

2.Cephalochordata.

These animals were evolved from common ancestor.

2. Urochordata or Tunicata:

The notochord is only present in the tail of the larva ( hence named urochardata) and disappears in the adult.  The notochord occurs only in the tail of larva and disappears in the adults called retrogressive metamorphosis

1.      They are marine,

2.      mostly sessile, filter–feeders.

3.      Body is enclosed in a leathery test or tunic sheath, composed of tunicin (cellulose) so called tunicates.

4.      The nerve chord (neural tube) is present in the larva, but is replaced by a single dorsal ganglion in the adult.

5.      The gill slits or stigma are numerous, persist in the adults and open into the atrium, instead of opening to the exterior. There are no true gills but are called branchial basket.

6.      Circulatory system is of open type. Blood consists of Venadocytes.

7.      Excretory system is lacking.

8.      Asexual reproduction occurs by budding.

Examples: Herdmania, Salpa, Doliolum etc.

 3. Cephalochordata:

1.       These are also marine and filter-feeders in feeding habit.

2.      The notochord extends up to the cephalic or head region and persists throughout the life.

3.      The nerve chord persists throughout the life, but no brain is formed.

4.      Excretion occurs by solenocytes.

5.      The gill slits are numerous and persist in the adults.

6.      True gills are absent.

7.      The body wall consists of myotomes.

8.      Tail persists throughout the life.

Hemichordates :

1.   All hermichordates are marine.
2. Some are solitary and slow moving, others are sedentary and colonial.
3. Body is stout and unsegmented, and has a worm–like or vase–like
4.   Three distinct regions namely proboscics, collar and trunk are present.
5.   They are bilaterally symmetrical and triploblastic.
6.   Locomotory appendages are absent. The collar may bear arms and tentacles.
7. Body wall consists of single layered epidermis and musculature of smooth longitudinal fibres.
8. Their body consists of a true coelom
9. Digestive tract is complete.
10. Feeding on microorganism and are ciliary or filter feeder
11.   Proboscis contains a hollow out growth from the gut, called the buccal diverticulum or stomochord and was regarded as notochord in the past.
12. Respiration by pairs of gill slits or through the general body surface.
13. Circulatory system includes a dorsal heart, two main longitudinal vessels; a dorsal and a ventral, interconnected by small lateral vessels and sinuses.
14. Blood is colorless and without corpuscles.
15. Nervous system is diffuse consisting of an epidermal plexus of nerve cells and nerve fibers.
16. Excretory system comprises of a proboscis gland, or glomerulus,
17. Sexes are separate or hermaphrodite.
18. The gonads may be in several pairs or only one in pair.
19. Fertilization is external or internal.
20. Asexual reproduction may occur.
21. Development may include a free-swimming tornaria larva.

Examples:  Balanoglossus (the acorn worm), Cephalodiscus, Rhabdopeura etc.