The document discusses the larval forms of mollusca, detailing three primary types: trochophore, veliger, and glochidium larvae. It describes the characteristics, developmental stages, and ecological roles of each larval type, highlighting their adaptive features for survival in aquatic environments. The conclusion notes the prevalence of free-swimming larvae in environments where adults are fixed and discusses the dependency of internal parasites on hosts for their development.

1. A PROJECT ON
“THE LARVAL FORMS OF
MOLLUSCA”
BY-
Sambit Bhai Patel

2. CONTENTS
1. INTRODUCTION TO MOLLUSCS _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ page 1
2. TYPES OF LARVAL FORMS IN MOLLUSCS _ _ _ _ _ _ _ _ _ _ _ page 2
3. TROCHOPHORE LARVA _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ page 3
4. VALIGER LARVA _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ page 5
5. GLOCHIDIUM LARVA _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ page 7
6. CONCLUSION _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ page 9
7. SOURCE _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _page 10

3. introduction
• Mollusca = mollis (L.) = soft bodies.
• 1st used by Aristotle to the cuttle fish .
• Second largest phylum of animal
kingdom.
• Animal having soft bodies with in shell
and ventral muscular foot.
• Found in all possible habitat except in the
air.
• Most of them are marine.
• Dioecious or monoecious.
• Fertilization either internal or external.
• Development is either direct or indirect.
• Direct development -- no larval stage ,
indirect development– with larval stage.

4. Types of larva
• 3 types of larva are found in Mollusca.
TROCHOPHORE VELIGER GLOCHIDIUM

5. Trochophore larva
• Pear shaped.
• Measures about 0.5 mm in length.
• Circle of preoral cilia.
• Prototroch or velum divides the body into
two unequal parts.
• Upper one consist of prostomium.
• Lower part bearing mouth and anus.
• Preoral part is large and convex.
• Upper end consist of apical tuft.
• Prototroch is the swimming organ.

6. • Comprises mouth.
• Oesophagus.
• Stomach.
• Intestine (mesenteric).
• Sides of the mouth two ciliated elevations
present each consisting of a single cell.
• Lower end bearing a bunch of cilia called
“telotroch”.
• Generally planktonic and feeds on tiny
suspended particle, living or dead.
• Body divided in 3 regions -- pretrochal
-- pygidium
--growth zone.
• Pretrochal region consist of apical plate,
prototroch , the area surround the mouth.
• Pygidium consist of telotroch and the area
surrounding the anus.
• Growth zone lies between the mouth and
telotroch.
• The trochophore larva later develops into a
veliger larva

7. Veliger larva
• Preoral ciliate area.
• Velum begin to protrude on both sides as a
bilobed flap.
• Very delicate.
• Anterior end of the larva provided with eyes
and Tentacles.
• Larva has a shell.
• Velum is developed from the prototroch of
the trochophore.
Page 5

8. • Larval heart and kidney present which is
situated at the anterior end of the body
immediately behind the velum.
• Statocyst and gill-rudiments present.
• Long cilia of the velum function
Locomotion.
• suspension feeding.
• Alimentary canal is complete.
• Anus is shifted to anterior side.
• Foot usually bearing an operculum.
• During development of veliger larva
torsion occur.

9. Glochidium larva
• Glochidium larva enclosed by two valves.
• Each edge of which bears a hook.
• Shell valves cover a larval mantle.
• Bears four groups of sensory bristles.
• Rudimentary foot is present.
• Attached a long adhesive thread called
“Byssal thread”.
• Neither mouth nor anus.
• Measures from 0.1 mm to 0.5mm .

10. • Highly modified for a parasitic existence on
fish.
• They clamp on the body and other part of
the body.
• The larval mantle contains phagocytic cells
that feed on the tissue of host and obtain
nutrition for development.
• This period lasts for about 10-30 days.
• In the mean time the parasite is surrounded
by the overgrowth of skin of fish forming a
cyst.
• Some of the larger freshwater molluscans
may produce as many as 30,00,000
glochidia.

11. CONCLUSION
• Free-swimming larva are usually formed
when the adult is fixed.
• Internal parasites generally have a stage
which may be called larval stage in which
they are transferred either by active or
passive migration to a new host.

12. source
• Google
• Modern textbook of
zoology invertebrate book
by R.L. Kotpal.
• Biology of Mollusca book
by D.R. Khanna &P.R. Yadav.
• Wikipedia.
• www.biologyjunction.com/
mollusca
• ecoursesonline.iasri.res.in/m
od/resource/view.php?id=85576
• Invertebrate zoology book
by barnes,fox,ruppert.