Sponges
Porifera (pore bearers)
Sponges have a very simple body plan: imagine a flower vase with innumerable pores in the wall (a very leaky vase). A special type of cells cover the inner surface, called [choanocytes] collar cells, cells with cylindrical or conical collars surrounding one flagellum per choanocyte. The wave-like motion of flagella drives water through the sponge's body. Food particles stick down on the collar. The food particles are transported by protoplasmic flow towards the cell body. There they are enclosed into the cytoplasm by phagocytosis. Thereafter the choanocytes pass it on to an amoebocyte, which on its turn passes it on to other cells.
The phylum Porifera is composed of three distinct groups,
- the glass sponges, skeleton composed of silicon dioxide
- the horn sponges, like the bath sponge, skeleton composed of protein fibres (spongin)
- the calcareous sponges, including coralline tropical reef sponges: skeleton composed of calcium carbonate
Venus’ flower basket
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Euplectella aspergillum Owen, 1841
A glass sponge: Its skeleton consists of an intricate lacework of siliceous spicules. They live in the deep seas of the Philippines and Japan. They house a couple of shrimps. Those shrimps clean the inside of the basket, the sponge provides food for the shrimp. [It is also speculated that the bioluminescent light of bacteria harnessed by the sponge may attract other small organisms, which the shrimp eat.]
When the Venus flower basket is small, tiny shrimps swim in and out of it. One male and one female make it their territory, their home. However, as the Venus Flower Basket grows it seals off the open upper end, and at the same time the shrimps grow, so that they cannot swim through the pores at the side of the Venus flower basket. As this happens, the pair will spend the rest of their lives inside that Venus flower basket. Their offspring is small enough to leave the basket through the pores.
To the Japanese this is a symbol of eternal love and being happily married forever. A Venus flower basket is sometimes given as a wedding present in Japan because of this beautiful symbolism.