Porifera (sponges)

On this page, types of sponge cell, canal systems, skeleton.

There are four Classes of sponge, Class Hexactinellida, Class Calcarea, Class Demospongiae, Class Sclerospongiae


Body with cells arranged around pores, canals and chambers for the passage of water.
Skeletal structure of spicules and/or spongin.
All aquatic, mainly marine.
Filter feeders.
Sessile adults; sessile, planktonic larvae.
Sexual and asexual reproduction.
Gas exchange is by diffusion.
Ability to grow continuously.


An organ system.
A nervous system.
An immune system.
Much co-ordination between cells.
Tissue organization, they have some but it is very restricted.
Symmetry, though some are radially symmetrical.
A true body cavity or gut.

Overview of the sponges

The sponges are an ancient group with a fossil record back as far as the Precambrian. There are about 10,000 known species of marine sponges, and about 150 freshwater sponges, with 5 British freshwater species. They are relatively abundant in all marine waters at all depths. They vary in size from a few millimetres to over two metres across.

Their cells are structured around a system of pores, chambers and canals through which water is moved by the action of the flagellae of the choanocyte cells. The large pores called oscula (singular - osculum) are water outlets, and the small pores called ostia are water inlets.

Sponges have the ability to grow continuously, but lack an immune system. However when they are exposed to x-rays, even in amounts 100 times greater than the human lethal dose, the sponges do not develop tumours. Also their cells continue to divide throughout their entire life. (bioRxiv,doi.org/f4dp)

Types of sponge cell

Pinaocytes. These are thin, flat epithelial cells covering the exterior surface and some interior surfaces. When they are located around pores they help to regulate water flow.

choanocyte sponge cell

Choanocytes (see above) are responsible for maintaining the current of water through the sponge, and for processing food particles in the water current. The water current brings in oxygen and food, and removes waste. Eggs and sperm are also carried out of the sponge on this water current. The rate of water flow can be regulated by changing the size of the osculum. Choanocytes line the central cavity and small cavities in the canal systems (more below). In the drawings below the choanocyte layer is shown as a thickened black line. Each choanocyte has one flagellum ringed by a contractile collar (see above).

Archaeocytes digest food particles passed from the Choanocytes, secrete spicules, spongin and collagen.

Canal systems

There are three types of canal systems, in the following drawings the arrows show the direction of water flow:

asconoid canal system

two synconoid canal systems

leucaniod canal system

Sponge skeleton

Porifera spicules

The skeleton of spicules and spongin provides the support to keep the pores open. The spicules can be either siliceous or calcareous of a variety of shapes (see above), and can be used in identifying species. The spongin is a form of collagen.

Regeneration. Sponges have the amazing ability to regenerate an entire individual from just a few cells.

Class Calcarea

As the name suggests sponges in this class have spicules of calcium carbonate. The spicules are either free or fused. These sponges tend to be relatively small, mostly less than 10 cm, and tubular or vase-shaped. All three types of canal system occur in this class. There are about 100 species, mainly marine in water no deeper than 1000m.

Class Sclerospongiae

This is a small group of sponges that resemble corals. They are usually found in dark tunnels in coral reefs. The skeleton consists of siliceous spicules and spongin on a thick basal layer of calcium carbonate. All in this class have the leuconoid canal system.

Class Demospongiae Class Hexactinellida