PHYLUM-CTENOPHORA
Phylum—Ctenophora (Ktene-comb; phors—bearing) or
comb jellies or sea Walnuts are exclusively marine forms.
The term ‘Ctenophora’ was coined by Georges Cuvier. It
includes about 50 species.
General Features
Some important general features of phylum—Ctenophora are
discussed below
(i) Habitat and Habit These are of exclusively marine
forms. They are found solitary, pelagic or free-
swimming.
(ii) Body Organisation They are diploblastic,
acoelomate with tissue grade of organisation. Body is
soft, delicate, transparent and gelatinous, like jelly
fishes without segmentation.
(iii) Body Symmetry They are biradially symmetrical.
The arrangement of comb plates gives the appearance
of radial symmetry, the tentacles and branching of
gastrovascular canals show bilateral symmetry.
(iv) Digestive System Digestion is both extracellular and
intracellular. Skeletal, circulatory, respiratory and
excretory systems are absent.
(v) Respiration and Excretion These processes occur
through general body surface.
(vi) Nervous System It consists of nerve cells with an
aboral sense organ called statocyst for balance.
(vii) Reproduction and Development They are
hermaphrodite or monoecious. Gonads are
endodermal. Fertilisation is generally external.
Development is indirect, e.g., Ctenophora,
Pleurobranchia.
Note
Bioluminescence (the property of a living organism to emit light) is
well-marked in ctenophores.

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).

Animals have three basic body plan as given below
(i) Cell aggregate body plan Cells are loosely arranged
and do not organise to form tissues, e.g., sponges.
(ii) Blind sac body plan Body has a cavity that helps in
digestion with a single opening, the mouth, e.g.,
coelenterates.
(iii) Tube within tube body plan It is found in higher
animal groups from nematodes to chordates. The body
consists of two openings, i.e., one for ingestion and
one for egestion.
It is further divided into two types
(a) Protostomous plan (Proto-first; stoma—mouth)
found in non-chordates like flatworms,
roundworms, annelids, molluscs and
arthropods. In this, mouth is derived from the
blastopore of the embryo and anus is formed
later.
(b) Deuterostomous plan It is found in
echinoderms, hemichordata and chordates. In
this, anus is derived from the blastopore and
mouth arises at the opposite end.

In animals like annelids, arthropods, molluscs,
echinoderms and chordates, organs have associated to
form functional systems, where each system is
concerned with a specific physiological function. This
pattern is called organ system level of organisation.
Organ systems in different groups of animals exhibit
various patterns of complexities.
For example, the digestive system in platyhelminthes
has only a single opening to the outside of the body that
serves as both mouth and anus and is hence called
incomplete. A complete digestive system has two
separate openings, mouth and anus.

The difficulties that will be faced if common
fundamental features are not taken into account are as
follows
(i) The interrelationship among different animals cannot
be traced.
(ii) The picture of all animals at a glance cannot be
projected.
(iii) The development of other animal species cannot be
made.
(iv) There are numerous animal species which show great
variety of life. The various common fundamental
characters are taken into account for the classification
of these animals. If this has not been done, it is not
possible to deal with every living form separately at
individual level. Therefore, a classification has to be
based on the common fundamental characters for the
sake of study of diversity in living forms.