1. Torsion is a twisting of parts of the body that occurs during development in gastropods. This causes the mantle cavity and pallial complex to shift to the front of the body. 2. Specifically, the visceral hump and pallial complex undergo a 180 degree anti-clockwise rotation, bringing the mantle cavity and anus to the front. This changes the gastropod body from bilateral to asymmetrical. 3. Theories on the evolution of torsion suggest it provides advantages for both larvae in escaping predators and adults in stability and water flow. Detorsion may partially or fully reverse the effects of torsion in some groups.

1. TORSION IN GASTROPODA
Bollapalli Ashok Kumar
M.Sc.,B.Ed.,M.A(Lit)CSIR-NET,(Ph.D)
LECTURER IN ZOOLOGY
KRK Govt Degree & PG College
ADDANKI-523201
Phone:9652929696,9441635264
Email: ashokkumarzoology@gmail.com

2. INTRODUCTION
• All the molluscs show bilateral symmetry
except the Gastropods which are
asymmetrical body with mantle cavity lying
anteriorly and the shell and the visceral mass
coiled spirally and directed posteriorly
• This is because of torsion or twisting of the
body or visceral mass at the time of
development

3. DEFINITION AND SITE OF TORSION
• గాస్ట్రో పొ డ జీవులలో TORSION ఒక ప్రత్యేకమైన లక్షణం
• ఈ దృగవిశ్ేంలో అంతరంగ సముదాయం మరవయు పాల్లియల్ కుహరం
180 డిగరీలులో rotation జరుగుత ంది
• Due to this movement the mantle cavity and pallial complex
is brought to the front side in the adult
• The head and foot remain fixed and do not take part in
torsion
• The part behind the head that includes the neck through
which oesophagus, rectum, aorta and visceral nerve loop
undergo torsion

4. • Coiling and torsion are two different
processes that occur in gastropods
• Coiling takes place in the shell and
accordingly the body of the animal which
lives inside the shell
• Torsion is limited to certain parts of the
body and it is no way related to general
coiling of the shell and the body

5. • Coiling of the shell is of two types namely
• Planospiral
• Conispiral
• Plano spiral in which the whorls of the shell
lies outside the preceding coil
• Conispiral in which the whorls of the shell
coils round a pillar like structure called
columella.
• Conispiral coiling took place in evolution
because it gives more balance of visceral
cavity and solve problems caused due to
torsion

6. • In course of evolution gastropods became asymmetrical in
body except in head region
• The reason is that the shell increases in size to
accommodate the growing body and it topples to one side
• So the mantle cavity and pallial complex shifted forward
from posterior position
• The foot of the gastropods grew and expanded while viscera
expanded into a big hump on the back
• If this hump grows vertically into a broad cone, it hinders the
locomotion of the animal
• So the viscera becomes spirally coiled to compactly placed

7. • As the growth advances the viscera becomes
voluminous, it looses its position and leans more to
one side
• This causes unequal growth of the body and right
side develops more than that of left side
• This requires adjustment of structures of the body.
• So in the larval stage a torsion occurs in the parts
like visceral hump and the pallial complex in 180
degrees around a vertical axis

8. • It occurs in between head and anus in anticlockwise in the
dorso ventral direction
• As a result the mantle cavity is shifted forward and makes
the anus lie by the side of mouth
• In this torsion, the original left ctenidium topples over the right
side.
• The intestine is thrown into the loop and pleuro visceral
connectives form into a figure of “8” by undergoing twist
• The right connectives become the supra intestinal while the
left becomes infra intestinal
• This twisting of nerves is known as streptoneury

9. • The above explanation is an old one and now it
is not totally accepted
• Because in Acmaea the whole process is
completed in 2 or 3 minutes
• Now it is supposed that it is due to asymmetrical
position and pull of the larval retractor muscles
which run from velar lobes to shell
• They are present only on right side whose
contractions causes torsion

10. • In this process only narrow neck of the
larvae twisted . So that everything present
between head and anus undergoes an anti
clock rotation through an angle of 180
degrees in a dorso ventral direction
• So by torsion, originally dorsal or exogastric
shell becomes ventral or endogastric.

12. THOMSON’S(1958) EXPLANATION
• Complete or 180degrees rotation , achieved by muscle contraction
is limited to Acmaea only
• 180degrees rotation is achieved in two ways .i.e. first 90 degrees
movement is brought about by the contraction of larval retractor
muscle and the remaining 90 degrees movement is by difference in
growth. It is seen most common way in molluscs like Haliotes and
Patella B
• 180 degrees rotation may occur only by differential growth as in
Vivipara
• Rotation by differential growth processes with anus coming to a
position appropriate to the adult state as in Aplysia
• Torsion no longer recognisable as a movement of Visceropallium,
the organs in the post torsional position from their first appearance
as in Adalaria

13. EFFECTS OF TORSION
• Effects of torsion were first realised by SPENGEL in 1881.
• He coined two terms regarding this phenomenon
• Streptoneury- nerves undergo a twist in the form of figure ‘8’ and
• Euthyneura- nerves become straight after torsion
• The effects are as follows
• Displacement of mantle cavity
• Change in position
• The alimentary canal forms loop due to torsion
• Chaistoneury
• Exogastric position of visceral sac and shell becomes endogastric due to
torsion
• Change in position of various organs

14. DISPLACEMENT OF MANTLE
CAVITY
• Due to increase in the length of foot, the
mantle cavity and pallial complex from
posterior region are shifted forward and
mantle cavity opens behind the head.

15. CHANGE IN POSITION
• Before torsion the Anus, Ctenidia and renal
openings face backwards and they face forwards
after torsion
• The auricle which is placed behind the ventricle
before torsion comes to front of it after
• Organs of viscera of original right side becomes
placed on left side and vice versa

16. CHAISTONEURY
• Long , uncoiled pleuro- visceral nerve
connectives are twisted to form a figure of
“8”.
• So that the right pleurovisceral becomes
supra intestinal nerve and
• The left pleuro visceral becomes the infra
intestinal nerve

17. •Organs like ctenidia, kidney,
osphradium etc present on left side
move to right side and persist
•While the organs on right side atrophy
and disappear thus losing the original
symmetry

18. THEORIES ON TORSION
•Garstang’s theory
• Morton’s theory

19. GARSTANG’S THEORY
• It is useful to larva rather than adult as per Garstang
• He explained that the mantle and head are at the posterior to the foot
before torsion and hence when attacked by enemies the delicate parts
like mantle and foot are exposed to them
• But after torsion the mantle cavity become anterior so that the sensitive
parts .i.e. head and velum could be withdrawn first and then foot
• When operculum sealed the aperture of the shell the animal fell down to
bottom and escaped from the enimies

20. OBJECTIONS TO GARSTANG’S THEORY
• Many pelagic larvae of bivalves are successfully living pelagic
life in which case why not a gastropod larvae is a big question
• Secondly, the movement of velar cilia is under the control of
N.S. and their movement can be stopped at any moment
without retracting into the shell
• In some animals like Haliotes only Partial torsion i.e. 90
degrees occur during larval period at which time head can not
be withdrawn first into mantle cavity.
• The remaining 90 degrees of torsion occurs only after the
larvae settles at the bottom

21. MORTON’S THEORY
• This is quite opposite to Garstang’s theory
• According to this theory, the advantage is to the adult than the
larvae
• He says that due to torsion the bulky pallial complex comes
nearer to the substratum thus increasing the stability of animal
• When the animal moves up the stream free flow of water into the
mantle cavity could not take place because the water should
enter from the posterior side of the animal which is difficult
because the water flow is against the movement of the animal.
• But after torsion as the entire pallial complex is shifted anteriorly
the inflow becomes easier.

22. • Due to torsion organs like osphradium
come closer to the head and along with
the tentacles of the head it can
continuously monitor the water that is
entering the body so that fresh and pure
water can only be directed in.

23. DETORSION
• When the changes occurred during torsion are
reversed, it is called detorsion
• This causes straightening of the nerves and the
group in which it occurs is called Euthyneura
• Because of this detorsion, the pallial complex goes
back to the posterior end, the auricle occupies the
place behind the ventricle
• And the visceral loop untwines and becomes
symmetrical and hence the nerves become
straightened. So the external symmetry is brought
back

24. • The extent of detorsion varies in different groups
• In groups like Opisthobranchiata and Pulmonata the
detorsion is incomplete
• So the visceral loop remains partially in twisted
condition
• The anus and ctenidium are directed laterally instead
of forwards.
• However in Opisthobranchiata group(Aplysia) there is
complete detorsion
• In groups like Nudibranchiata, the mantle cavity,
visceral hump, external shell and ctenidia are lost
completely Eg: Doris, Aeolis etc