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The sessile sponges, which had appeared by the Early Cambrian (and possibly substantially earlier), are the simplest multicellular animals alive today. Being widespread in aquatic, mainly marine, environments (with approximately 9,000 species), they constitute the phylum Porifera, which means 'pore-bearing'. This name refers to their main characteristic, a unique water canal system consisting of pores and canals that has no direct counterpart in other metazoans and is used for filter feeding (although some sponges in food-poor environments have become carnivorous). Water with food particles enters the sponge through pores in the side of the body (these are called ostia) and then exits via the osculum, a larger opening at the top. Choanocytes, specialised cells with a central flagellum that are strikingly similar to the unicellular choanoflagellates, generate a flow of water and serve to capture most of the food. Sponges had long been assumed to be monophyletic, being subdivided into demosponges (Demospongiae), calcareous sponges (Calcarea) and glass sponges (Hexactinellida), but recent molecular data have provided evidence for their paraphyly. The proposed new taxonomy would suggest that such a water canal system was in fact primitive to all animals and lost in eumetazoans.
The poriferan body shape is variable and, although sometimes very regular (e.g. in glass sponges), not symmetrical. Sponges consist of different cell types embedded in a central jelly (the mesohyl), but cell differentiation is reversible, providing these animals with remarkable regenerative capacities. For structural support, sponges possess a skeleton of spicules (organic or mineralised) that in some species are made of silica, while consisting of calcium carbonate in others (with the mode of calcification being strongly convergent). The skeleton of demosponges also contains stiffening but relatively soft fibres made of spongin, a form of collagen. Spicules provide some interesting examples of evolutionary convergence. Glass sponges possess siliceous spicules that are remarkably similar to the optical fibres used in telecommunication. They could act as light guides that channel light into the body, probably to supply symbiotic algae. Mysteriously, however, glass sponges are usually found below the photic zone, so it has been suggested that the spicules could function as "reverse eyes", broadcasting the sponge's bioluminescence and thus potentially attracting symbiotic shrimps. In one deep-sea species, the single basal spicule is up to 3 m long and might represent a photoreceptive system that potentially employs cryptochromes. These proteins are convergently used in light sensitivity and probably also help birds detect the Earth's magnetic field. Biological optical fibres might have also evolved in a syllid annelid, and a component of the human retina has fibre-optic capacities as well. Demosponges show examples of the convergent evolution of microscleres, small supporting spicules, where similar types of microscleres developed independently from different original spicule types.
The simplicity of sponges is particularly reflected in their lack of nerves, muscles and sense organs. These animals can, however, react to touch (although their reactions are not exactly fast) and show some sort of coordinated behaviour. Regular contractions of the body ("sneezing") serve to expel water, probably to prevent clogging of the feeding system. Despite the lack of any sort of nervous system, in glass sponges electrical impulses run through the tissue! These impulses are, for example, generated in response to tactile stimuli and induce the animal to stop feeding. Remarkable as this is, it is not unique. Such electrical signalling without a nervous system has also been demonstrated in more advanced animals as well as in plants, possibly as an alternative to nerves.
Most sponges are hermaphrodites, producing free-swimming flagellated larvae that then disperse. While some release their fertilised eggs into the water (oviparity), in the majority of species the eggs are retained until hatching (ovoviviparity). In demosponges, ovoviviparity is probably ancestral, while oviparity has been acquired secondarily in at least two separate lineages. There are also several sponges that can reproduce asexually by budding, and many freshwater and some marine species are able to form gemmules, resistant stages that can regenerate when conditions are favourable again.
The cell biochemistry of sponges is complex and includes biotoxins (some sponges poison their neighbours to make room for growth!), substances potentially useful to humans, such as anti-inflammatory agents, and pigment granules. Many sponges are very colourful, but the colour can also be due to photosynthetic endosymbionts, mainly cyanobacteria in marine species and green algae in freshwater species. One species of freshwater sponge, however, lives in symbiosis with a yellow-green alga, and it has been suggested that this symbiosis has evolved convergently.