PHYLUM
PORIFERA
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(porus-opening;
fere-to bear)
It is the
group of the most primitive animals popularly called sponges.
These were
recognised as animals by ELLIS.
The term
porifera was introduced by ROBERT GRANT.
The study of
sponges called PARAZOOLOGY.
Most of them are marine and remain attached to rocks. Some lives in fresh water.
Spongilla is a freshwater sponge.
Important
characteristic features:
• They are sedentary in habitat and are solitary or colonial. They
possess a cellular level of organisation. Cells aggregate to form body plan.
• Most of them are asymmetrical animals while some are radially
symmetrical.
• They are pore bearing animals and their body is perforated by
numerous openings or pores called OSTIA.
• Body wall: Have two loosely differentiated layers of
cells. An outer dermal layer of flat cells called pinacocytes and an
inner layer of flagellated cells called as collar cells or choanocytes.
They are diploblastic.
• Water transport system(water canal system):
Body is in the form of a cylinder which encloses a large cavity called spongocoel. Spongocoel opens out by a narrow opening called osculum. This pathway helps them in food gathering, respiratory exchange, and removal of waste.
Ostia or pores (water enters) → spongocoel (central cavity) → osculum or pores (water goes out).
• Choanocytes:
Each ostium leads to a canal which leads to spongocoel. The canal is
lined with flagellated collar cells called choanocytes. It brings food and
oxygen and carries away excretory and reproductive products.
Each
choanocyte has long flagellum projecting from the free end of each cell. The
lower part of the flagellum is surrounded by a transparent collar. In simple
sponges the canal is not well developed. In such cases the ostia open in to the
spongocoel through hollow cells called porocytes and the spongocoel is
lined with choanocytes.
• In between
the two layers of the body there is a non – living gelatinous matrix called
mesenchyme. The mesenchyme has a number of amoeboid cells, spicules, and
sponging fibres.
Amoeboid
cells: wander through the matrix carrying food between the cells.
Spicules:
are siliceous or calcareous structures formed of silica or calcium carbonate.
Sponging
fibres: are silk elastic structures.
• Skeleton: it is made up of spicules and sponging fibres.
• Sponges are holozoic (take solid food) in nutrition. The
digestion is intracellular.
• Reproductive
system: they are hermaphrodites (sexes are not separate)
Reproduces
both sexually and asexually.
Asexual
reproduction takes place
by fragmentation and budding.
Fresh water
sponges during unfavourable conditions produce buds known as gemmules which
remain dormant till favourable season.
Sexual
reproduction: during
sexual reproduction some amoeboid cells become egg or sperm.
Fertilisation
is internal
and the development of larva is indirect. After fertilisation the zygote
develops in to a larva. The larva of sponges are ciliated and free
swimming.
They are of
2 types: amphiblastula and parenchymula.
• All sponges possess great power of regeneration.
• Examples
for common sponges are Leucosolenia, Spongilla (fresh water sponge), Sycon
(scypha), Euspongia (bath sponge), Eupelectella (venus flower
basket).Its dried skeleton is used as a costly marriage gift in japan. It is
considered a symbol of union till death. Hyalonema (glass rope
sponge), Cliona (sulphur sponge), Chalina (dead man finger
sponge), Proterion (neptunes cup) are some more examples.
Classification of Porifera
Class I:
Calcarea (calcareous sponges. eg: Sypha, Leucosolenia, etc.)
ClassII:
Demospongia (skeleton of sponging fibres. Largest class of sponges. Eg:
Euspongia, Spongilla, etc.
ClassIII:
Hexactinellida (glass sponges with silicious spicules. eg: Euplectella,
Hyalonema, etc.
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