SPONGES
Sponges are a diverse group of sometimes common types, with about 5000 species known across the world. Sponges have cellular-level organization, meaning that that their cells are specialized so that different cells perform different functions, but similar cells are not organized into tissues and bodies are a sort of loose aggregation of different kinds of cells. This is the simplest kind of cellular organization found among parazoans. Other characteristics of sponges include a system of pores and canals, through which water passes. Water movement is driven by the beating of flagellae, which are located on specialized cells called choanocytes (collar cells). Sponges are either radially symmetrical or asymmetrical. They are supported by a skeleton made up of the protein collagen and spicules, which may be calcareous or siliceous, depending on the group of sponges examined. Sponges capture food that is brought close by water currents created by the choanocytes. Food items are taken into individual cells by phagocytosis, and digestion occurs within individual cells. Reproduction by sponges is by both sexual and asexual means. Asexual reproduction is by means of external buds. S Sexual reproduction takes place in the mesohyl. Male gametes are released into the water by a sponge and taken into the pore systems of its neighbors in the same way as food items. Spermatozoa are "captured" by collar cells, which then lose their collars and transform into specialized, amoeba-like cells that carry the spermatozoa to the eggs. Sponges are found in virtually all aquatic habitats, although they are most common and diverse in the marine environment.
| Development: Radial cleavage leads to a blastula with an opening (called a stomoblastula), which turns itself inside-out before swimming free from the parent sponge |
| Body cavity (between endo- and ectoderm): Not applicable |
| Digestive System: None |
| Circulatory System: None |
| Excretory System: None |
| Nervous System: None |
| Reproductive system: Mostly hermaphroditic, but do not make eggs and sperm simultaneously |
| Claim to Fame: Sponges are the only animals with their type of organization: a pile of cells that use a unique kind of cell (choanocytes) to sweep water into channels and filter out food |
Diagram of the collar cells
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Tubular Sponges
