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For the glory of our creator, The entirely Merciful, The Compassionate.
Department of Zoology
Mid Term Assignment
Course Code: ZOL- 102 Course Name: Principles of Animal Life- II
Due Date: 27 April, 2020 Submission Date: 27 April, 2020
Course Teacher: Prof. Dr.Muhammad Ali kanwal Student Name: Momina Bakhtawar
Student ID: BZOF19M001 Session: 2019-2023
Program: BS Zoology Semester: II
Regular/ Self Support: Regular Main Campus/ Ex-PPP: Main Campus
Title of the Assignment: Water Vascular System(WVS) in Brittle Star.
Word count: 2,014
Marks Awarded by Teacher:
TABLE OF CONTENTS
Abstract
Introduction
Topic Explanation
Conclusion
References
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ABSTRACT: Every Organism in this world is phylogenetically somehow related to one another
but it is also a fact that each organism has distinctive characteristic which make it unique to
enhance the nature mysteries and to open new ways to Scientific Biological Research . This
assignment traces the Water Vascular System in Brittle Star; an Invertebrate Animal related to
Phylum Echinodermata.
Water Vascular System is one of the unique characteristics of whole phylum Echinoderms(can be
related to chordates due to mesodermal skeleton and deuterostomous origin). My goal is to show
that even this system varies in the animals of the same phylum and to show that Nature is not as
simple as it seems, there is always some complexity in simplicity and to find answer we have to
simply concentrate to meet new mystery, as a straight line in ECG machine is not the end.
INTRODUCTION:
Generally we know that Water vascular System is series of water filled canals in Echinoderms
used in locomotion, attachment and feeding. This speciose system is modified to perform different
functions even in different organisms of same class .To understand Water Vascular System in
Brittle star it is necessary to evaluate the lineage of Brittle stars(Ophiuroidea). Brittle Stars,
basically belongs to Zoology of Marine Invetebrates. Morphologically, it is star shaped animal
having five rays (in some species more than five rays have also reported) emerging from central
disc (ranges in size from 1-3cm). The undulating appearance of rays is like a snake ( Figure 1 ) ,
crawling on some surface as the word Ophiuroidea( Gr. Ophis, snake + Oura, tail) depicts.
Figure 1 Ophioderma rubicundum, the Ruby Brittle Star in St. Lucia out at night.
It is believed that brittle stars evolve after the bottleneck effect of Permian-Triassic mass extinction
about 250 million years ago although originated over 400 million years ago. In systematics, brittle
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stars have caused difficulty for taxonomists to trace their evolutionary relationship within the
phylum indicating their diversity in function and structure as well. One of the unique characters of
brittle star is that it cast-off its rays (autotomy) if get entangled in danger and thus can easily
escape, hence called brittle (splitting) star.
It is commonly known that locomotion of echinoderms is because of tube feet; extensions of water
vascular sytem. It is probably true only for class Asteroidea(star fish), (earth worm like
movements) , as it is observed that brittle star move due to contraction and relaxation of ray
muscles thus giving whipping flagella like appearance and not all rays participate in moving. Thus
it is considered that locomotion was not the main function of water vascular system. Here the
question arises that; then what is the purpose of water vascular sytem in brittle star? And how it
differs from WVS of other classes structurally as well?. Thus to answer these questions, it is
important to study Water Vascular System in Brittle star, which seems to be very simple but
depicting complex beauty of Nature.
WATER VASCULAR SYSTEM:
Water Vascular system, as the name is indicating that the system consisting of vessels or veins
conducting or circulating water or fluid. It is somewhat like hydraulic system using pressurized
liquid to perfom functions, and according to Pascal , pressurized fluid is uniform in all directions.
Embryologically, it is considered as modification of coelom ( a fluid-filled body cavity formed from
the splitting of lateral plate mesoderm during embryonic development) and internally ciliated
probably to control backflow of liquid. The main parts of water vascular system are tube feet
(podia), fluid-filled flexible muscular extensions of canal sytem which are modified to perform
different survivalship functions like feeding,respiration, sometimes excretion and generally known
for their locomotory function. Similarly, the most common terms related to WVS are ring canal,
radial canal, stone canal,lateral canals and a sieve- like plate called the Madreoporite ( a round
structure which is like a porous door controlling inlet and oulet of water and filtering sea water).
It is common query that what is the composition of fluid present in water vascular system. It mostly
contains sea water and fluid of albuminous nature. Presence of leucocytes is also reported, we can
say fluid is also serving as antimicrobial agent. Existence of red corpuscles is also recorded in an
Ophiuroid specie. Moreover potassium and Amoebocytes are also present. Coelomocytes are also
part which play important role in molecular biology.
Some associated structures such as Polian vesicles(series of sacs) and Tiedemann’s bodies are also
part of water vascular system which function as reservoir of fluid and water filter body
respectively.
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Generally, stone canal is considerd as a pump which conducts water from Madreoporite to ring
canal which is divided in different symmetrical directions to form radial canals. Radial canals give
rise to lateral canals which bear tube feet. Radial canal is sometimes present outside the skeleton as
in Asteoroids and thus called open Ambulacral system. To serve as locomotory organs, tube feet
possess ampullae and suction cup. Ampullae is a muscular structure whose contraction causes tube
feet to extend and elongate and thus suction cup present at distal end also contracts and creates
vacuum which helps to get attach to substrate and this coordinated process occurs in all tube feet
which may be more than 2000 in a single echinoderm. Sometimes these tube feet may secrete sticky
mucous which helps in climbing and crawling and acts as adhesive material.
This was the general structure of a typical water vascular system. Now, we will have a look on how
this system serves in brittle stars and modifications which make brittle stars unique in their own to
enhance complex beauties of nature.
Water vascular system of Brittle stars:
The main point to note is that water vascular system of brittle stars is not used for locomotion,
although sometimes it can secrete sticky substances to serve as climbing material. Ambulacral
system is closed thus no ambulacral grooves are present. Unlike Asteoroids, the madreoporite of
ophiuroids is on the oral surface i-e near mouth and these openings can be many thus it is generally
not easy to recognize madreoporite. Tiedimann bodies are also absent.The most important
distinguishing character of brittle stars is that tube feet in brittle stars lack ampullae and suction
cups thus not involved in locomotion. Their tube feet end in blind distal end which is sharp and is
surrounded by tentacles. This characteristics help them to feed easily to catch their prey from
cervices as well by using their pointed sticky tube feet. As we know ophiuroids are the most diverse
and modern among echinoderms thus this modification of tube feet play major role in evolutionary
history as their arms look like snake and snake don’t possess feet and move by rowing. Isn’t it
interesting?
Role of WVS in feeding:
The main function of water vascular in brittle stars is feeding. Contraction of polian vesicles are
responsible for the extension of tube feet. They have different feeding strategies. Tube feet secretes
large amounts sticky substances which help in feeding and attachement. After capturing prey the
tube feet pack their food in food balls and transport to mouth through arms canal system. It is
usually the characteristic of suspension-feeding species of brittle stars. Among the prey animals are
more commonly planktonic copepods, diatoms, Foraminifera and detrietal drifting material in
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bottom of rocks and coral reefs. Sometimes they use their pointed tube feet to dig the surface or to
displace other items to extract organic material from sediments.
Figure 2
Figure 3
In the above pictures, we can see the oral view of brittle stars. It is where mouth is present and thus
madreoporite is also present near.In figure 3, tube feet can be seen.
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Role of WVS in respiration:
It is interesting to know that water vascular system in brittle stars also play role in respiration. The
thin ciliated epidermal lining serves as diffusion membrane. Gases can easily diffuse and moreover
osmotic pressure of system also helps in respiration. It is observed that a respiratory pigment
haemoglobin that is contained in coelomocytes is present in Water Vascular system. The presence
of coelomocytes is very important from evolutionary point of view. Although due to well-developed
ossicle system, oxygen consumption rate is lower but important to meet the basic needs thus oxygen
enters in tube feet through water vascular membrane. The tube feet of brittle stars possess
unidirectional valve, thus oxygen diffuses above the valve and then enters coelomic fluid and
exchange of gases takes place. However, brittle stars also possess Bursae in body disc for additional
oxygen uptake.
In brittle stars it is also observed that tube feet may sereve as sensory structures . It is considerd
that the tube feet contains cells that contain genes associated with photoreceptor cells. Echinoderm
are highly sensitive to light and in species of brittle stars colour changes have seen in day night
changes. It is due to migration of pigmented cells in skin. Due to migration, light sensitive neurons
are exposed and thus colour changes take place.
It is also reported that water vascular system sometimes also serve as excretory system showing
that how diversified this system is.
Working of Water vascular system:
As already discussed Madreoporite(sieve like plate) allows sea water to enter the system and
position of madreoporite in different classes of echinoderms is different and not only this, this
position can be different in species of same class. After that water enters ring canal through stone
canal and the structure and composition of stone canal( named as it contains calcium carbonate)
also varies a little bit in every class. Ring canal makes a ring around esophagus. Thus water enters
radial canals radiating from ring canals and exerts pressure on tube feet to perform different
functions. It can also be seen that water and coelomic fluid moves like counter current mechanism
and membranes act as dialyzer.
This mechanism is generally same in all classes with few exceptions in structure and mechanism
thus we can say that every specie is unique in its own.
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Figure 4 Cross section of arm of brittle star. Note the absence of suction cup in tube feet and their
blind ends.
Ophionereis reticulata, an Ophionereididae
It is observed that this specie of brittle star uses tube feet for climbing and locomotion as well.
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An overview of Brittle star.
Note the madreoporite presence on oral surface.
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CONCLUSION:
From above all discussion, I am reached at a point to say that Water vascular system seems to be
very simple but it is very complex and diverse int its structure as well as function. We cannot depict
for what function it is actually for, but scientists believe that the function of this system is mainly
feeding probably and it is modified in different classes to perform several functions and locomotion
is result of search for food , a survivalship ability. Actually it depends on the conditions and
environment and thus animals adapt themselves according to such conditions to survive and
reproduce. Nature seems to be simple but it is very diverse to concentrate. Similarities in different
classes and groups opens amazing facts but there is a point in every organism where it diverges
from other and it is complexity of nature. This was amazing to search on brittle stars.
REFERENCES:
Zoology by Harley and Miller 5th edition (Pg# 242,243,246,247,253).
Invertebrate Zoology ( Pg# 498,499, 503,504,505)
https://www.amazon.com/Echinodermata-Progress-Molecular-Subcellular-
Biology/dp/3540244026
https://books.google.com.pk/books?id=EtkqDwAAQBAJ&printsec=frontcover&dq=australi
an+echinoderm&hl=en&sa=X&ved=0ahUKEwinqeDv5YbpAhXMy4UKHYdxCT8Q6AEIK
DAA#v=onepage&q=australian%20echinoderm&f=false. (pg 25 – 36)
https://books.google.com.pk/books/about/Physiology_of_Echinoderms.html?id=qw3LBAAA
QBAJ&redir_esc=y (pg 21,22,23,28,29,30)
https://www.bioscience.com.pk/topics/zoology/item/567-water-vascular-system-in-different-
echino-derms.
https://animaldiversity.org/accounts/Ophiuroidea/.
https://books.google.com.pk/books?id=a58KgI2niE4C&pg=PT117&dq=water+vascular+sys
tem+in+brittle+stars&hl=en&sa=X&ved=0ahUKEwjrqtuz64bpAhUQDxQKHVJaC-
QQ6AEIJTAA#v=onepage&q=water%20vascular%20system%20in%20brittle%20stars&f
=false
https://collections.museumvictoria.com.au/species/15892.
http://www.microscopy-uk.org.uk/mag/indexmag.html?http://www.microscopy-
uk.org.uk/mag/artaug17/rh-echinoids-2.html.
https://www.newworldencyclopedia.org/entry/Brittle_star
