PHYLUM PORIFERA: SPONGES
Characteristics of Phylum Porifera:
o Multicellular; body an aggregation of several types of cells
differentiated for various functions, some of which are organized
into incipient tissues with some integration
o Body with pores (osita), canals, and chambers that form a
unique system of water currents on which sponges depend for
food and oxygen
o Mostly marine; all aquatic
o Radial symmetry or none
o Outer surface of plat pinacocytes; most interior surfaces lined
with flagellated collar cells (choanocytes) that create water
currents; a gelatinous protein matrix called mesohyl contains
amebocytes of various types and skeletal elements
o Skeletal structure of fibrillar collagen (a protein) and calcareous
or siliceous crystalline spicules, often combined with variously
modified collagen (sponging)
o No organs or true tissues; digestion intracellular; excretion and
respiration by diffusion
o Reactions to stimuli apparently local and independent in cellular
sponges, but electrical signals in syncytial glass sponges; nervous
system probably absent
o All adults sessile and attached to substratum
o Asexual reproduction by buds or gemmules and sexual
reproduction by eggs and sperm; free-swimming flagellated
larvae in most
The Origins of Multi-cellularity:
While sponges are multicellular, their organization is quite distant from
other metazoans. A sponge body is an assembly of cells embedded in
an extracellular matrix and supported by a skeleton of minute needle-
like spicules and protein.
Origin of Metazoa (Animals):
Choanoflagelletes are solitary/colonial aquatic eukaryotes, with each
cell carrying a flagellum surrounded by a collar of microvilli. Beating of
the flagellum draws water into the collar, where microvilli collect tiny
Many choanoflagelletes are sessile and attached to hard surfaces.
Choanoflagellete cells strongly resemble sponge feeding cells called
Adult sponges have simple bodies; they are aggregations of several
different cell types, including choanocytes, held together by an
extracellular matrix. Most sponge bodies are not symmetrical, but some appear radial. A sponge body has neither a mouth nor a digestive
Phylum Porifera: Sponges:
Sessile sponges draw food and water into its body instead. A sponge
uses a flagellated ‘collar cell’, the choanocytes to move water. The
beating of many tiny flagella, one per choanocyte, draws water past
each cell, bringing in food and oxygen, as well as carrying away
wastes. The sponge body is designed as an efficient aquatic filter for
removing suspended particles from the surrounding water.
Although their embryos are free swimming, adult sponges are always
The skeletal framework of a sponge can be fibrous and/or rigid. When
present, the rigid skeleton consists of calcerous or siliceous support
structures called spicules. The fibrous part of the skeleton comes from
collagen protein fibrils in the intercellular matrix of all sponges.
Collagen comes in several types of differing in chemical composition
and form. One form of collagen is spongin.
The presence of photosynthetic organisms inside the sponge led some
to propose that spicules were able to transmit ight into the body.
3 classes of porifera:
Calcera have spicules of crystalline CaCO3 with 1, 3 and 4 rays.
Hexactinellida are glass sponges with 6-rayed siliceous spicules, where
the 6 rays are arranged in 3 planes at right angles to each other.
Demospongiae have a skeleton of siliceous spicules that develop
around an axial filament, or sponging fibers, or both.
A fourth clase, Homoscleromorpha, contains sponges that lack a
skeleton/have siliceous spicules without an axial filament.
Form and Function:
Sponges feed primarily by collecting suspended particles from water
pumped through internal canal systems. Water enters canals through a
multitude of tiny incurrent pores in the outer layer of cells, a
Incurrent pores = dermal pores.
Inside the body, water is directed past the choanocytes, where food
particles are collected on the choanocytes collar. The collar comprises many fingerlike projections, called microvilli. The use of the collar as a
filter is one form of suspension feeding.
The smallest particles, accounting for about 80% of the particulate
organic carbon, are taken into choanocytes by phagocytosis.
Choanocytes may acquire protein molecules by pinocytosis. Two other
cell types, pinacytes and archaetocytes, play a role in sponge feeding.
There are 3 main designs for a sponge body, differing in placement of
In the simplest as asconoid system, the choanocytes lie in a large
chamber called the spongocoel.
In the syncoid system, the choanocytes lie in canals.
In the leuconoid system, the choanocytes occupy distinct chambers.
Types of Canal Systems:
Asconoid sponges draw water inside through dermal pores by the
beating of large numbers of flagella on the choanocytes. These
choanocytes line the internal cavity called the spongocoel. As the
choanocytes filter the water and extract food particles from it, used
water is expelled through a single layer osculum. This design has distinct
limitations because choanocytes line the spongocoel and can collect
food only from water directly adjacent to the spongocoel wall.
Asconoids are small and tube-shaped, because if the spongocoel
were large, most of the water and food in its central cavity would lie in
‘dead cavity’ inaccessible to choanocytes.
Asocnoids occur only in class Calcera.
Synconoid sponges have a tubular body and single osculum, but the
body wall, which is really the spongocoel lining, is thicker and more
complex than that of asconoids. The lining has been folded outward to
make choanocyte-lined canals. Folding the body wall into canals
increases the surface area of the wall and thus, increases the surface
area covered by choanocytes.
Most of the water in a canal is accessible to choanocytes. Water
enters the syconoid body through dermal ostia that lead into incurrent
canals. It then filters through tiny openings, or prosopyles, into the radial
canals. Here food is ingestedby the choanocytes. The beating of the
choanocytes’ flagella forces the used water through internal pores, or
apopyles, into the spongocoel. Food capture does not