The simplest two phyla (major groups) of animals, Placozoa with its single genus and Porifera, known as sponges, make up the subkingdom Parazoa ("alongside animals"). Members of these phyla lack tissues organized into organs and most have an indeterminate form. They also lack bodily symmetry of any kind. One group of sponges is believed to have given rise to all other animals (Collins and Waggoner, 2001).
Aristotle (History of Animals 5.16 - line number!!) classified sponges as animals, citing popular reports of their ability to contract when disturbed as evidence of their sensitivity. However, sponges have also been regarded as plants, solidified sea foam or even the homes of marine worms. Only in the last 200 years have all scientists come to accept that they are animals.
Sponges are now recognised by scientists as bona fide individuals. Because a sponge lacks internal organs, everything that happens inside its body (e.g. moving, breathing, eating, and reproducing), occurs at the cellular level (Stewart, 1999). The cells in a sponge's body are relatively independent of one another, and are also totipotent (capable of developing into a complete organism or differentiating into any tissue in the organism). Not surprisingly, sponges were formerly thought to be colonies of independent one-celled animals rather than individuals - Lindquester (1999) considers a sponge to be "in many ways ... more a colony ... than a multicellular organism". However, most biologists now agree that each sponge is "an individual, made up of cells" (Stewart, 1999). Each individual is "recognizable as a mass of cells, canals and mineral skeletal structures, surrounded by a common layer of flattened cells" (Soest, 2002).
Although sponges have internal skeletons, they possess no definite organs, no circulatory systems, and no definable nervous systems. Despite their simplicity, sponges contain analogues of features found in more complex animals - a circulatory system (most sponges' bodies contain a system of canals lined with cells whose tail-like flagella pull in water containing tiny food particles); a bloodstream (sponges also contain some free-ranging cells that spread nutrients); and muscles (some cells in sponges contract or open up to regulate the size of the outer pores) (Stewart, 1999).
Ten of the estimated 10,000 species of sponges are now known to be able to crawl - albeit at the rate of a few millimetres a day. As they move, these sponges re-arrange their internal structure, cell by cell. This ability was only discovered by time-lapse photography, showing sponges moving at 1200 times their natural speed (Stewart, 1999).
Although sponges are sensitive to light, poison and touch, their adaptive behavior is limited. Poking causes sponges to contract. However, this contraction takes about 20 minutes to spread from the point where the sponge is poked, through the entire body. The spreading of the contraction is slow because without nervous systems, it depends only on cell to cell connections and chemical diffusion (Grossman, 1996), although certain glass sponges, which consist mostly of a single enormous cell with countless nuclei that is stretched over the sponge's skeleton, can shut down quickly when disturbed, and researchers have recently found evidence of electrical activity moving along the cell membrane (Stewart, 1999).
Aside from Placozoa and Porifera (sponges), there are two other small phyla of animals (Rhombozoa and Orthonectida) which appear to have evolved independently of the lineage leading to the remaining 33 phyla of eumetazoa, or "true" animals. They will not be discussed in this thesis.
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