Chordata

Phylum Chordata is one of the most well-known and diverse phyla in the Animal Kingdom. It includes animals ranging from simple marine organisms to highly complex vertebrates like humans.

1.0Introduction

• Organ system level of organisation
• Bilaterally symmetrical, triploblastic, and coelomate animals
• Notochord
• Dorsal hollow nerve cord
• Paired pharyngeal gill slits
• Closed circulatory system
• Post anal tail

2.0Characters of Chordates

• All the chordates possess four diagnostic characters either in the embryonic or adult stage. 
• Notochord: It is a solid un-jointed, stiff but flexible rod-like structure situated on the dorsal side between the dorsal hollow nerve cord and the alimentary canal. 
• Dorsal Hollow Nerve Cord: The nerve cord of chordates is always hollow and lies dorsal to the notochord. 
• Pharyngeal Gill Slits: All the chordates have at some stage of life, a series of paired narrow openings, the gill slits on the lateral sides of the pharynx.
• Tail: It is a post-anal part of the body.Which is reduced or absent in many adult chordates. 

Other Characters of Chordates

• These include bilateral symmetry, three germinal layers, segmentation, organ-system level of organisation, cephalization, coelom, endoskeleton, complete digestive tract, special organs for respiration and excretion, closed circulatory system, separate sexes, gonads with gonoducts and without asexual reproduction. 

Phylum Chordata can be divided into two groups: 

• Acrania (Protochordata) 
• Craniata (Euchordata) which show contrasting characters. 

Acrania (Protochordata)

• All are marine, small, primitive or lower chordates. 
• No cranium, jaws, vertebral column, paired appendages. 
• About 2,000 species. 
• The Acrania is divided into three subphyla- Hemichordata, Urochordata and Cephalochordata. 

3.0Subphylum I. Hemichordata

• The body is divided into 3 regions- proboscis, collar and trunk. 
• Notochord doubtful, short confined to proboscis and non-homologous with that of chordates. 

Class 1. Enteropneusta

• Body large and worm-like. Gill-slits numerous and paired. 
• Alimentary canal straight. Acorn or tongue worms. 
• Enteropneusts include 3 families, 15 genera and 70 species. 
• Examples- Balanoglossus, Saccoglossus, Ptychodera. 

Class 2. Pterobranchia

• The body is small and compact. 
• Gill-slits one pair or none, never U-shaped. 
• Alimentary canal U-shaped. 
• Pterobranchs include 2 orders, 3 genera and 20 species. 
• Examples- Cephalodiscus, Rhabdopleura.

Class 3. Planctosphaeroidea

• Transparent, round and specialised tornaria larva possessing extensively branched ciliary bands. 
• Alimentary canal L-shaped. 
• Planctosphaera pelagica is a representative of this class. 
• This form is supposed to be specialised tomaria larva of some unknown hemichordate. 

Class 4. Graptolita

• The fossil graptolites (e.g., Dendrograptus) were abundant in Ordovician and Silurian periods. 
• These are often considered as an extinct colonial class of Hemichordata. 
• Their tubular chitinous skeleton and colonial habits show an affinity with Rhabdopleura. 

4.0Subphylum II. Urochordata or Tunicata

• Notochord and nerve cord in tail only in minute free-swimming tadpole larva. 
• Adult sac-like, covered with tunic (test) often transparent; usually no notochord nerve cord reduced to ganglion; no coelom, segmentation or nephridia. 

Class 1. Ascidiacea

• Sessile tunicates with scattered muscles in tunic. Solitary, colonial or compound. Gill-slits many; tunic well developed, permanent. 
• Ascidians or sea squirts. Include 3 orders, 12 families, 37 genera and 1,200 species. Examples- Herdmania, Ciona and Molgula. 

Class 2. Thaliacea 

• Free-swimming or pelagic tunicates with circular muscle bands in tunic. 
• Salpians and chain tunicates. 
• Include 3 orders, 5 families, 9 genera and 30 species. 
• Examples- Salpa, Doliolum, Pyrosoma. 

Class 3. Larvacea or Appendicularia 

• Tiny, transparent, free-floating. Adults retain many larval features including tails. Only two gill-slits. 
• Tunic not persistent. 
• Include 2 orders, 2 families, 5 genera and 3 species. 
• Examples- Oikopleura, Appendicularia. 

5.0Subphylum III. Cephalochordata

• Notochord and nerve cord present throughout life along the entire length of the body. 
• Include 1 class, 1 family, 2 genera and 30 species. 

Class Leptocardii

• Body fish-like, segmented with numerous gill-slits. 
• Free-swimming and burrowing. Lancets. 
• Examples- Branchiostoma (= Amphioxus), Asymmetron. 

Group B. Craniata

• Aquatic or terrestrial, usually large-sized, higher chordates or vertebrates with distinct head, a vertebral column, jaws and brain protected by a cranium or skull. 
• The craniata includes a single subphylum Vertebrata. 

6.0Subphylum IV. Vertebrata

• Notochord supplemented or replaced by a vertebral column or backbone consisting of overlapping vertebrae. 
• Body divisible into head, neck, trunk and tail. Usually dioecious. 
• Vertebrata is the largest chordate subphylum including about 45,000 species.