Traditionally, sponges have been regarded as a monophyletic group defined by several synapomorphies (Hooper, Van Soest, and Debrenne 2002), including the presence of:
- an aquiferous system with external pores
- mineral spicules
- high cellular mobility and totipotency
The latter feature is often considered to be the most important (Lévi 1999). Indeed, the sponge body is unique among animals because it continuously remolds itself to fine-tune its filter-feeding system. The constant rearrangement of the body is accomplished by the amoeboid movements of cells inside the sponge and their change from one differentiated form to another.
Although often considered immobile, sponges also display several behavioral patterns (resulting from coordinated movements of cells), including crawling, production of filamentous body extensions and body contractions (Nickel 2004). It is also often mentioned that sponges lack many characteristics associated with other animals, including a mouth, sensory organs, organized tissues and neurons and muscle cells, which are otherwise ubiquitous in Metazoa. It is difficult to say, however, whether the lack of aforementioned features represents a primitive condition of sponges or a secondary loss due to their sedentary and water-filtering lifestyle. Indeed, a recent study has shown that the homoscleromorph sponges possess several characteristics thought to be absent in sponges, including the presence of true epithelia (Boury-Esnault et al. 2003). Another study has found that although sponges do not have neurons, their genome contains most of the components needed to build a post-synaptic protein scaffold that is essential for neural impulse transduction in other animals (Sakarya et al. 2007).
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