This phylum consists of organisms which are considered as lower chordates. It consists of a huge range of organisms and also forms the major division of phylum chordata. These organisms are also known as acraniata as they do not possess a true skull. The main difference between vertebrate and protochordata is the absence of backbone in the organisms of protochordata. The different subphyla of phylum protochordata are urochordata and cephalochordata. Earlier, group hemichordate also comes under the phylum protochordata. But, nowadays hemichordates are considered as a separate phylum.

What is protochordata?

The word “protochordata” means primitive chordates. This phylum consists of organisms which are considered as lower chordates. It consists of a huge range of organisms and also forms the major division of phylum chordata. These organisms are also known as acraniata as they do not possess a true skull. Organisms of this phylum possess notochord in one point of life cycle.

Characteristics of protochordata:

• Habitat: Protochordates are all aquatic, mostly marine.
• Symmetry: The body of the organisms of protochordata is bilaterally symmetrical which means it can be divided into two equal halves when a plane is passed longitudinally.
• Organisms are triploblastic, which means they possess the outside ectoderm, inner endoderm and in the middle, mesoderm. Organisms are coelomates. Coelomate means it possesses true coelom, surrounded by mesoderm.
• Level of organisation: Organisms of this phylum possess organ-system level of organisms.
• At one point of life, the body of organisms consists of a long, rod-like structure which provides support known as notochord.

Classification of protochordates:

Hemichordata:

• The organisms of this subphylum are aquatic, mostly marine.
• A number of protochordate organisms are solitary but some live in colonies.
• The body of organisms under phylum protochordates are cylindrical, stout and unsegmented. 
• The body of the organisms is divided into proboscis, trunk and collar.
• The collar of the body possesses tentacles and arms.
• Organisms possess a complete digestive system.
• Respiratory system of the organisms includes general body surfaces and gills for respiration.
• Circulatory system of the organisms consists of a heart which possesses two longitudinal vessels. Blood of circulatory systems does not possess any corpuscles or pigment. 
• The proboscis gland in the organism acts as an excretory organ in case of phylum protochordates.
• Organisms possess united or separate sexes.
• Fertilization in the organisms of phylum protochordata is either external or internal.

E.g., Balanoglossus (tongue worm), Rhabdopleura, Cephalodiscus.

Urochordata:

• The organisms of this subphylum are aquatic, mostly found in marine environments.
• Organisms are filter-feeders and sessile.
• The organisms under this subphylum are also called tunicates as their body remains surrounded by a leathery sheath which is known as tunicin.
• In these organisms, the notochord appears in the tail of the organisms in the larval stage which remains absent in the adult. This type of phenomenon where larvae are much more developed than that of adults is known as retrogressive metamorphosis.
• In the larva, neural tube is present whereas in adults dorsal ganglion is present.
• In these organisms, the respiratory system consists of gills.
• The circulatory system of these organisms is open type that means, the tissues get washed directly in blood.
• The excretory organs in these organisms are absent.
• Reproduction in these organisms occurs by budding.

E.g., Herdmania, Selpa

Cephalochordata:

• The organisms of this subphylum are aquatic, mostly found in marine environments.
• Organisms are filter-feeders.
• In this phylum, organisms possess notochord throughout the body which extends from head to tail.
• The tail and nerve cord also stays with the organisms throughout life.
• Solenocytes act as excretory organs in case of organisms belonging to cephalochordata.
• Respiratory organs of the organisms are gills which are linked to the atrium.
• The body wall consists of myotome muscles.

E.g., Amphioxus

Conclusion:

Protochordata mainly refers to primitive types of chordates. They possess notochord in any time of the life cycle in the organisms of this phylum. Protochordates are all aquatic, mostly marine. Organisms are triploblastic, which means they possess the outside ectoderm, inner endoderm and in the middle, mesoderm. Organisms are coelomates. Coelomate means it possesses true coelom, surrounded by mesoderm. It consists of a huge range of organisms and also forms the major division of phylum chordate. These organisms are also known as acraniata as they do not possess a true skull. The article also contains different sub-phyla which come under protochordata. Those sub-phyla include hemichordate, cephalochordata and tunicate.