Protochordate , Cephalochordate

 

   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 consist of three sub-phyla which are as follows.

1. Urochordata

2. Cephalochordata.

These animals evolved from common ancestors.

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 the 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 adults, and open into the atrium, instead of opening to the exterior. There are no true gills but are called branchial baskets.

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 habits.

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

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

4.      Excretion occurs by solenocytes.

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

6.      True gills are absent.

7.      The body wall consists of myotomes.

8.      Tail persists throughout life.

Hemichordates :

1.   All hemichordates are marine.
2. Some are solitary and slow-moving, and others are sedentary and colonial.
3. The body is stout and unsegmented, and has a worm-like or vase–like
4.   Three distinct regions namely proboscis, 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. The body wall consists of a single-layered epidermis and a musculature of smooth longitudinal fibers.
8. Their body consists of a true coelom
9. The digestive tract is complete.
10. Feeding on microorganisms and are ciliary or filter feeders
11.   Proboscis contains a hollow-out growth from the gut, called the buccal diverticulum or stomochord and was regarded as a notochord in the past.
12. Respiration by pairs of gill slits or through the general body surface.
13. The 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. The nervous system is diffuse consisting of an epidermal plexus of nerve cells and nerve fibers.
16. The excretory system comprises of a proboscis gland, or glomerulus,
17. Sexes are separate or hermaphrodites.
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.

                                          

Class – Pisces

1.  The presence of gills for respiration is an important characteristic of fishes.
2.  Their skin is covered with scales/plates. 
3. They lay eggs [oviporous].
4. Some fishes are ovi-viviparous.
5. Presence of paired and unpaired fins.
6.  fertilization is internal. 
7. presence of air bladder act as an organ of buoyancy in some fishes.  
8. They obtain oxygen dissolved in water by using gills.
9. The body is streamlined, and a muscular tail is used for movement.
10. They are cold-blooded and their hearts have only two chambers.
11. Some fish skeletons are made entirely of cartilage [Chondrichthyes], such as sharks, 
12.  some with a skeleton made of both bone and cartilage [Osteichthyes].

CLASS: Chondrycthys / Elasmobranch

GENERAL CHARACTERS

1. They are marine animals with streamlined bodies and have cartilaginous endoskeletons. 
2. The mouth is located ventrally.
3. The notochord is persistent throughout life.
4. Gill slits are separate and without operculum (gill cover).
5. The skin is tough, containing minute placoid scales.
6. Their jaws are very powerful.
7. These animals are predaceous [shark].
8. Due to the absence of air bladder, they have to swim constantly to avoid sinking.
9. Heart is two-chambered (one auricle and one ventricle).
10. Some of them have electric organs (e.g., Torpedo) and some possess poison sting (e.g., Trygon).
11. They are cold-blooded (poikilothermous) animals, i.e., they lack the capacity to regulate their body temperature.
12. Sexes are separate. In males pelvic fins bear claspers.
13. They have internal fertilization and many of them are viviparous [give birth to young ones].
14. Examples: Scoliodon (Dogfish), Pristis (Sawfish), Carchaiodon (Great white shark), and Trygon (Stingray).