Protochordates

Or Acraniata

Subphylum: Urochordata

Week 3

Assist. lec. Sawsan S. Hameed

Biology Department

Tishk international University

2021-2022

Domain Eukarya

Kingdom Animalia

Phylum Chordata

Sub phylum

UrochordataSub phylum

Cephalochordate

Sub phylum

Vertebrate

Protochordates

Or Acraniata

Higher chordates

Or Craniata

• The Urochordates are exclusively marine animals living on the substratum (bed rock).

• The most familiar member of the group is Hardmania; it possesses two siphons for the entry and exit of water.

• Characteristically they have a larval stage which undergoes retrogressive metamorphosis giving rise to the adult animal.

• Urochordates show all the basic characters of chordata at one or the other stage of life, they are supposed to be lower than Cephalochordata but higher than hemichorates from evolutionary point of view.

Subphylum I: Urochordata (tail-chordates)

Also called Tunicata

(Gr., oura= tail , L. chorda= chord

General characters

Small size 0.25-250mm, free swimming

Body is covered by protective tunic (covering membrane).

Notochord only in tail region and its depend on the presence or

lost of the tail

Dorsal nerve cord is degenerate into nerve ganglia in adult

Pharyngeal gill slits persistent, as the animal respire through it

Most species with two holes (openings), incurrent (mouth) and

excurrent (openings)

Mostly hermaphrodite & Fertilization is external

Subphylum I: Urochordata (tail-chordates)

Figure 1. Urochordate metamorphosis

Larval

More than 2000 species belong to 4 classes

Sub phylum

Urochordata

Class AscidaceaClass SorberaceaClass ThaliaceaClass Larvacea

These animals also called sea squirts

General character

Larval stage are free swimming with notochord, nerve

cord and gill slits in addition to long tail.

Adult are inactive or sessile - after retrogressive

metamorphosis, losing tail so notochord is also lost

nerve cord change to ganglion.

Gill slits persist in the pharyngeal region.

sexes are united (hermaphrodite)

Class Ascidiacea

Figure shows:

a) typical solitary adult tunicate.

b) Larval or tadpole life cycle phase

showing the notochord and the

dorsal nerve chord.

c) A typical mature colonial tunicate

showing a shared tunic and multiple

incurrent siphons (blue arrows) which

are always greater in number than the

excurrent siphons (pink arrows).

The class is divided into two orders.

Order 1. Enterogona

These ascidians bear one gonad in

the intestinal loop.

Neural gland is ventral to the

ganglion. Tadpole larva is seen.

Ex: Ascidia and Ciona.

Order: 2. Pleurogona.

In these ascidians, gonads are paired and are present in the atrial wall.

Neural gland is dorsal to the ganglion:

Ex: Herdmania, Botryllus.

Class Ascidiacea

Internal Structure of Ascidia

Test of Herdmania

Different types of tunicates

Herdmania External characters

is a marine and sedentary animal.

It is fixed to rocky sea by a flat

base.

When it is disturbed, it contracts its

body, and releases inner contents

through its apertures.

Hence it is called Sea squirt.

Phylum: Chordata

Sub-phylum:Urochordata

Class: Ascidiacea

Bluebell tunicate

General character

Adult larval-like, retain many larval

features including tail, notochord,

nerve cord

No metamorphosis, hence called

larvacea.

Gill slits in pharyngeal region also

persist throughout the life

Sex are united

Class II: Larvacea

Tail

Class Thaliacea

General character

Free-swimming and pelagic forms.

They are covered by transparent test.

The brachial and atrial apertures are placed at anterior and

posterior ends and pharynx is small.

Tunic with circular muscles

Adult lack notochord, nerve cord and tail region

Incurrent and excurrent opening opposite to each other, each at

one end of the body

Sex are united, reproduce by asexual reproduction by

budding.

Larval formed or absent

Tunic with circular muscle

Incurrent Excurrent

Thaliacea

The class Thaliacea is divided into three orders.

Order1. Doliolida (Cyclomyarla)

Barrel shaped body is completely covered by muscle bands

Pharynx is small.

Number of gill slits is less

Tailed larva is seen

ex Doliolum

Order2. Pyrosomlda

This order includes colonial forms.

Muscle bands are small and present at the ends.

Gill-slits are many.

Tailed larval stage is absent.

Ex: Luminescent colonial form.

Order 3. Salpida (Hemimyaria):

This order includes organisms whose body prism is like.

A pair of lateral gill slits are present

Doliolum

• Presence of retrogressive metamorphosis.

• Absence of segmentation.

• Because of these characters zoologists included these animals in a separate sub-phylum Urochordata.

• During recent years many zoologists regarded the tunicates as primitive and ancestral forms of chordates as a whole.

• The typical tunicate looks like a non-chordate animal.

If the life history of such animal is studied, the larval form reveals the chordate characters of that animal.

Urochordates differ from other chordates owing to the following characters:

Urochordate’s-resemblance with Chordate.

• Urochordates resemble the chorates owing to the following features-

• Presence of dorsal tubular nerve cord.

• Presence of notochord.

• Well developed pharynx with gill-slits.

• Presence of endostyle on the ventral side of the pharynx.

• Presence of atrium around the pharynx.

• Presence of post - anal tail with tail fin.

• Because of these chordate features tunicates are included in chordate’s group

Subphylum II: Cephalochordata Classification

• Domain: Eukarya (eukaryotes)

• Kingdom: Animalia

• Phylum: Chordata (chordates)

• Subphylum: Cephalochordata

Subphylum of the phylum Chordata.

Cephalochordate or lancelets, traditionally known as amphioxus

It has the five basic characteristics of chordates in simple form.

About 2.5 to 7 cm in length.

Amphioxus (Gr. Amphi, both ends, + oxys, sharp)

Subphylum II: Cephalochordata

Cephalochordata example; Amphioxus

General Characterstics of Cephalochordata

a) Two pairs of cerebral and several pairs of spinal nerves are present.

a) Presence of dorsal tubural nerve cord.

General Characterstics of Cephalochordata

q) Sexes are separate. Gonads are metamerically arranged and are without gonoducts.

r) Asexual reproduction never occurs.

s) Fertilization is external.

Classification And Systematic Position Of

Cephalochordata

Presence of a long notochord from anterior end to posterior end on the dorsal side, and

since it extends to the cephalic region, the group is called Cphalochordata.

The sub-phylum Cephalochordata includes a single class- Leptocardii, which has single

family Branchiostomidae.

The family contains only two genera Branchiostoma and Asymmetron.

Pallas described these animals first and named the Cephalochordatas Umax lancealatus.

In 1834, Costa described its chordate features. In 1836, Yarrel named it as Amphioxus.

The characteristics and body structure of cephalochordates can be described under

the following heads;

a) CHORDATE FEATURES OF CEPHALOCHORDATA

(AMPHIOXUS):

i. Presence of dorsal tubural nerve cord.

ii. Presence of a long notochord from anterior end to posterior end on the

dorsal side. Because, it extends to the cephalic region, the group is called

Cphalochordata.

iii. Gill silts are present in the Pharynx.

iv. Presence of post anal tail.

v. Presence of liver diverticulum.

vi. Development of hepatic portal system.

vii. Presence of myotomes which are useful for locomotion.

viii. Presence of dorsal, caudal and ventral fins.

b) PRIMITIVE CHARACTERS OF CEPHALOCHORDATA:

i. The excretory system contains protonephridia.

ii. In Chordata, the presence of solenocytes is not reported. But, in Amphioxus

solenocytes are associated with nephridium.

iii. Absence of heart and kidney.

iv. Absence of paired limbs or paired fins.

v. Absence of distinct head.

vi. Absence of distinct paired sense organs.

vii. Gonads are without gonoducts.

Special Characters Of Cephalochordata

i. Because of its ciliary mode of feeding, the pharynx is elaborated with many gill slits.

ii. Oral hood is well developed for ciliary mode of feeding.

iii. Because of its ciliary mode of feeding the atrium is very well developed. Thus,

Amphioxus shows some special characters which are developed because of its ciliary

mode feeding.

Figure 5. A slide of Amphioxus showing the mouth

Epipharyngeal groove

Endostyle

Gill slits

• Get a view of all of the structures.

• Identify the five chordates characteristics:

• Pharyngeal slits ‐ Look for the pharynx at the anterior end of the animal. The

slits run from anterodorsal to posteroventral. During filter feeding, water enters

through the mouth, passes through the pharyngeal slits, and exits through the

atriopore.

• Dorsal hollow nerve cord – This is a dark pink stripe that runs anterior to

posterior along the dorsal side of the animal.

• Notochord – This is a dark stripe that often stains a more orange color than the

nerve cord. It also runs along the dorsal side of the animal, located immediately

ventral to the nerve cord. It begins anterior to the nerve cord.

• Endostyle – The endostyle is a structure that is difficult to see in the whole

mount. It runs longitudinally along the ventral edge of the pharynx. In the whole

mount, it may appear as a darker pink stripe.

• Postanal tail – Locate the anus, which is ventral and posterior to the atriopore.

The part of the animal posterior to this is the post‐anal tail.

Draw the amphioxus slides. Label all the structures that you identify.

Draw all the provided picture to identify the tunicates species

Lab study: A slide of Amphioxus

References

For further reading please see:

Charles K. Weichert (2017). Elements of chordate anatomy. 3rd edition.

The McGraw−Hill Companies, New york.

Comparative anatomy | Definition, Examples, & Facts |

Britannicahttps://www.britannica.com › science › comparative-anatomy

Kardong, Kenneth V. (2019). Vertebrates: comparative anatomy, function,

evolution (8th edition). New York.

De Iuliis, G., & Pulerà, D. (2019). The dissection of vertebrates. 3rd

edition. Academic press. Elsevier, London.

Kenneth, S. S. (2017). The unity of form and function. 8th edition. The

McGraw−Hill Companies,. New york.

Comparative Anatomy. wwww.health.zone/