Protochordata refers to lower chordates that lack a true skull, broadly classified into three sub-phyla: hemichordata, urochordata, and cephalochordata. These organisms are primarily marine, exhibit bilateral symmetry, and possess a notochord at some life stage, with distinct classification and characteristics within each sub-phylum. Hemichordates are divided into classes like enteropneusta, while urochordates include tunicates, and cephalochordates, the most advanced, retain the notochord throughout life.

1. Protochordata: General characters and
classification up to orders
By,
Dr. Manoj K. Bangadkar

2. Protochordata
• The organisms belonging to the Protochordata are generally
known as the lower chordates.
• They don’t form a “proper” taxonomic group and are only
classified as such for convenience purposes.
• However, they do form a major division of Chordata.
• They are also known as Acraniata because they lack a true
skull.
• On the basis of the notochord present, they are divided into
three sub-phyla- Hemichordata, Urochordata and
Cephalochordata.

3. Characteristics of Protochordata
• They are generally found in marine water.
• Their body is bilaterally symmetrical, triploblastic, and
coelomated.
• At a certain stage of their lives, their body develops a long,
rod-like structure for support called notochord.
• They exhibit organ system level of organization.
• Eg., Herdmania, Amphioxus.

4. Classification of Protochordata
Protochordata
Hemichordata
(Half chordate)
Urochordata
(Tail cord)
Cephalochordata
(Head cord)

5. HEMICHORDATA
• Marine, Solitarily
• Body is divided into proboscis, collar, and trunk.
• Bilaterally symmetrical
• Primitive notochord restricted to proboscis only, thus called
stomochord.
• Triploblastic
• True coelom present
• Straight or U shaped gut with anus
• Nervous system normally diffuse and variable
• Open circulatory system
• Blood has no colour and corpuscles.
• Excretory organ- Glomerulus
• Filter feeding
• Four classes- Enteropneusta, Pterobranchia, Planctosphaeroidea,
Graptolita
• Eg., Cephalodiscus, Rhabdopeura

6. Classification of Hemichordata
Hemichordata
Class I-
Enteropneusta
Class II-
Protobranchia
Order I-
Rhabdopleurida
Order II-
Cephalodiscida
Class III-
Planctospheroidea
Class IV-
Graptolita

7. Class I – Enteropneusta (Acorn worms)
General Characters
• More than 70 species
• 2-2.5 m long
• Marine, shallow water, solitary
• Body divided into Proboscis, Collar
and Trunk
• Well developed gill slits and
stomochord
• Dorsal strand of nerve cells- precursor
of dorsal hollow nerve cord
• Reproduction- sexual
• Live in burrows
• Filter feeding

8. Class II – Pterobranchia
General Characters
• Tiny, deep sea, colonial
• No trace of dorsal nerve cord or
notochord
• Proboscis is modified into shield
• Collar is modified into tentacles
• Trunk is short and sac like
• Asexual reproduction by budding
is common
• One or no pharyngeal slits
• Body covered with protein
collagen

9. Class III – Planctosphaeroidea
General Characters
• Small, rounded, transparent
and pelagic larvae
• Larval body covered with
branched ciliary bands
• Elimentary canal ‘L’ shaped

10. Class IV – Graptolita
General Characters
• Exctinct class
• Fossil graptolites (e.g.
Dendrograptus) were
abundent in Ordovician and
Silurian period.
• Tubular chitinous skeleton
• Colonial habits

11. UROCHORDATA (TUNICATA)
• Tail chordate
• Most primitive of the chordates
• They are found in the marine environment.
• They are sessile and filter-feeders.
• They are also known as tunicates because their body is surrounded
by a leathery sheath composed of tunicin (cellulose).
• Retrogressive metamorphosis : The notochord appears in the larval
stage in the tail of the larva and disappears in the adult.
• Triploblastic
• U shaped gut, Coelom absent
• Hermophroditic
• Respiration occurs through gills.
• They have an Incomplete closed circulatory system.
• Haemocyanin as blood pigment
• The excretory organs are absent.
• They reproduce asexually by budding.
• Eg., Herdmania, Selpa

12. Clasification of Urochordata
Subphylum-
Urochordata
Class I-
Ascidiacea
Order I-
Enterogona
Order I-
Pleurogona
Class II-
Thaliacea
Order I-
Pyrosomida
Order II-
Doliolida
Order II-
Salpida
Class III-
Larvacea
Order II-
Endostylophora
Order II-
Polystylophora

13. Class I - Ascidiacea
• Sea squirts
• Notochord and post anal tail found in the larval stage only
• Marine, solitary or colonial, Sessile adults
• Tunic composed of an acellular matrix of tunicin
• Two opening- an inhelant and exhalent siphon
• Hermaphroditic
• Filter feeders
• Two orders- I) Enterogona and II) Pleurogona

14. • Order I- Enterogona: Body
divided into thorax and
abdomen, Neural gland ventral
to ganglion, Single gonad lying
behind intestinal loop, larva
with two sense organ (Ocelli
and Otolith), Ex. Ascidia
• Order II- Pleurogona: Body
compact (Undivided), Neural
gland dorsal or lateral to
ganglion, Gonad two or more
embedded in mantle wall,
larva with otolith, ex.
Herdmania

15. Class II - Thaliacea
• Small barrel shaped animals
• Filter feeders
• Inhalent and exhalent siphon at opposite ends of body
• Two generations of life cycle- one solitary and other form
chain like colonies
• Three orders- I) Pyrosomida, II) Doliolida and III) Salpida

16. • Order I – Pyrosomida :- No
free swimming larval stage,
ex. Pyrosoma
• Order II – Doliolida :- Larva
with notochord present, ex.
Doliolum
• Order III- Salpida :- Tailed
larva absent, ex. Salpa

17. Class III – Larvacea (Apendicularis)
• Larvaceans or Apendicularians
• Planktonic
• Oval trunk and long thin tail
• Tail contain notochord which retain all through the life
• Body covered with gelatimous layer but not tail.
• Two orders- I) Endostylophora and II) Polystylophora
• Order I - Endostylophora :
House bilaterally
symmetrical with two
apertures, pharynx with
endostyle, ex.
Appendicularia
• Order II – Polystylophora :
House bilaterally
symmetrical, single
aperture, pharynx without
endostyle, ex. Kawalevskia

18. CEPHALOCHORDATA
• Most advanced protochordates
• Notocord: Well developed, persist throughout the life and
runs from tail to head (Head cord)
• Gill slits: Numerous (Upto 100)
• Dorsal nerve cord
• Post anal tail
• Marine
• Fish like appearance
• External fertilization
• Metamerism found
• Closed blood circulatory system
• Haemocyanin pigment
• Single class- Leptocardii / Leptocardia

19. Class I – Leptocardii / Leptocardia (Small heart)
• No true heart- A single pulsating vesel
• Colorless blood
• No brain, organ of excretion and limbs
• Backbone is notochord
• Ex. Amphioxus
Figure: Amphioxus