PHYLUM-PORIFERA
Phylum—Porifera (Porous — pore; ferre — to bear) includes the
first and the most primitive group of multicellular animals
referred to as poriferans, i.e., pore bearing animals.
They are primitive, multicellular animals and are
commonly called sponges.
General Features
Some general features of phylum—Porifera are discussed
below
(i) Habitat They are mostly marine and some are
freshwater habitat, found in ponds and lakes, e.g.,
Spongilla.
Note
The study of poriferans is called Parazoology, Even though
they are multicellular, they do not have tissue grade of organisation.
(ii) Shape and Size The size of sponges ranges from 1 cm
to 1m in length. Some of them have vase-like
cylindrical shape and majority have irregular shape.
(iii) Symmetry The cylindrical form (e.g., Sycon) show
radial symmetry while, sponges showing irregular
shape and have no symmetry.
(iv) Body Wall The body wall contains outer dermal layer
or pinacoderm and inner gastral layer or choanoderm.
The pinacoderm contains flat cells called pinacocytes.
The choanoderm consists of spherical cells with
collar from which a flagellum emerges out. Hence,
they are also called flagellated cells or collar cells.
(v) Body Organisation Sponges show cellular level of
organisation. The cells are almost independent, i.e.,
cooperating very little with one another in
function.
(vi) Germ Layers These are diploblastic, i.e., having
ectoderm and endoderm.
(vii) Body Cavity The sponges have a large cavity
called spongocoel or paragastric cavity. It opens to
the outside by a terminal opening called osculum.
viii) Skeleton It is internal and includes inorganic
spicules and organic spongin fibres.
(ix) Canal System It is one of the most important
feature of sponges. Canal system is the network of
channels called canals, which connect the
spongocoel to the outside through ostia.
The canals are lined by choanocytes. It maintains a
constant flow of water from ostia to osculum
through spongocoel. It helps in nutrition,
respiration, reproduction and excretion.
(x) Digestion Digestion in animals of this phylum is
intracellular (inside collar cells) in nature.
(xi) Circulation The amoeboid cells transport food
from ingesting cells to other cells.
(xii) Respiration It occurs through the body surface by
diffusion.
(xiii) Excretion It also occurs through the body surface
by diffusion in the form of ammonia.
xiv) Sensory System Sponges lack sensory and nerve
cells, so they are poor in response to stimuli.
xv) Reproduction Sponges reproduce both asexually
and sexually. Asexual reproduction occurs by
budding or gemmules or internal buds. Sexual
reproduction involves the fusion of ova and
spermatozoa.
i) Fertilisation and Development Fertilisation is
internal i.e., sperms of one sponge are carried by
water current to the ova of another sponge for in
situ fertilisation. Development is mostly indirect
with larval stages called parenchymula
(Leucosolenia) or amphiblastula (Sycon).