Phylum Mollusca: Larval forms with structure

General Characteristics of Mollusca, Larval form with their Structure

2. Introduction

In mollusca, the development may be direct or
indirect. In the direct development, there will be no
larval stage and the young ones will hatch out form
the eggs which resemble their parents in general
appearance, except the size. The direct development
is seen in cephalopods. In indirect development, a
larval stage in majority of molluscan forms. Paludina
is a viviparous form. Three main larval forms are
observed in molluscan forms.

3. Cont….

Mollusca= Mollis (L.) soft bodies

Ist 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,

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 Near the apical cilia

Two ciliated elevations each consisting of a single cell

Bearing a bunch of cilia called telotroch

6. 
Comprises mouth

Stomodaeum

Oesophagus

Stomach

Intestine (mesenteron)

Statiolith sacs appear

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– Pretrochl, pygidium, growth zone

Pretrochal region consist of apical plate, prototroch, the area surround the
mouth
Cont….

8. 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 Tentacles

larva has a shell

Velum is developed from the prototroch of the trochophore

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

9. 
foot usually bearing an operculum

During development of veliger larva torsion occur

larval heart and kidney present

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
Cont….

11. 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 Byssal thread
Neither mouth nor anus
Measures from 0.1 mm to 0.5mm
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
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.

13. References

Modern Textbook of Zoology Invertebrate
by R. L. Kotpal

Biology of Mollusca
by D. R. Khanna and P. R Yadav

Invertebrate Zoology
by Branes, Fox, Ruppert.