Proto chordata

protochordate, any member of either of two invertebrate subphyla of the phylum Chordata: theTunicata (sea squirts, salps, etc.) and the Cephalochordata (amphioxus). Like the remaining subphylum of the chordates, the Vertebrata, the protochordates have a hollow dorsal nerve cord, gill slits, and a stiff supporting rod, the notochord, the forerunner of the backbone. The protochordates differ chiefly from the vertebrates in not having a backbone. Recent protochordates are thought to have evolved from the same ancestral stock as that which gave rise to the vertebrates.

Two main theories have gained general acceptance as to how the vertebrates may have evolved. One theory proposes that the ancestral form was sessile (attached), perhaps like a pterobranch but with an unspecialized larva. This larva adapted to an independent pelagic life and became sexually mature. Subsequently, the sessile stage was lost, and the vertebrates evolved from this free-swimming animal. The other, more recent theory postulates that the chordates evolved from a small fossil group called the mitrates.

Protochordates

For more information on protochordate groups, see amphioxus; larvacean; salp; sea squirt; tunic ate.

Protochordates

Phylum chordata is divided into two groups, Acrania and Craniata. Acrania include those chordates which lack cranium, jaws, vertebral column and paired appendages. The members of Acrania are generally referred to as Protochordates or Lower chordates. Most of the protochordates are fairly allied to vertebrates but none possess the full complement of chordate key characters and in some even the presence of notochord is doubtful.

General characters of Protochordates:

• They are simple and aberrant chordates.
• The true brain or skull is absent.
• The true heart and kidneys are absent.
• They lack jaws altogether.
• Vertebrate and paired appendages are absent.
• Peculiar type of embryonic development is found.
• They are all marine and small sized animals.
• Head and exoskeleton are not found.
• Coelom is enterocoelic.
• Pharyngeal gill-clefts are permanent.
• Endostyle is present.