Sensory and Nervous System The sensory and nervous system becomes much more complex as one moves through the phyla of the animal kingdom. From phylum placozoa to phylum Chordata, the sensory and nervous systems gradually get more and more involved. Phylum Placozoa consists of only one known species, Trichoplax adhaerens (Nielson, 2001, p. 48). Made up of only a few thousand cells, and generally only 2 mm across, this species does not even seem as though it should be considered part of the animal kingdom at all (Campbell, 2005, p. 639). This “flat organism creeps on algae” and has been found in warm waters from all over the world (Nielson, 2001, p. 48). The placozoans are thought to be a sister group to the eumetazoans because they have similar features, however, “the complete lack of a nervous system sets Trichoplax aside from eumetazoans” (Nielson, 2001, p. 50). Due to the fact that this animal “exhibits limited specialization and organization of cells,” placozoans do not have any sensory or nervous system (Sumich, 2004, p. 132). [pic] Phylum Porifera is more commonly known as sponges. “Sponges are simple, sessile animals that lack true tissues” (Campbell, 2005, p. 639). Sponges do not have a specific system or group of organs and cells that make up a nervous system. Although sponges lack a nervous system, epithelial cells play an important role in the conduction of stimuli (Floreano, 1999, p. 237). “There is no system of coordination, so the responses to stimuli are very simple” (Puranik, 2007, p. 108). The way the sponge’s body is arranged, as a loose arrangement of cells, makes the body “devoid of power of conductivity” (Puranik, 2007, p. 108). When a sponge undergoes a very strong stimulus, such as a cut, there is no transmission sent through the sponge for more than 3 to 4mm over the surface of the sponge. The osculum, a large opening in the sponge, is the most sensitive part, where the conductivity of the sponge is at its maximum. When the osculum is stimulated in some way, the “transmission quickly passes away from the opening” (Puranik, 2007, p. 108). One such example of the sensory and nervous system at work in the sponge was observed when an osculum collapsed when the inhalant water was polluted (Puranik, 2007, p. 108). The sponge underwent some sort of stimuli from the polluted water that caused the rest of the cells in the osculum to respond and collapse together. This same sort of response occurred when the sponge was exposed to a strong beam of light or touched by certain objects (Puranik, 2007, p. 108). In 1955, Tuzet and Parvans de Cecally “reported the occurrence of bipolar and multipolar nerve cells in the mesenchyme”, the space between the pinacocytes and choanocytes (Puranik, 2007, p. 108). This claim was neither accepted nor denied when reviewed in 1962 by Jones and 1965 by Horridge. So although there was some speculation about sponges having a sensory and nervous system, it is still widely accepted that they lack them, other than the characteristics previously discussed. [pic] Phylum Cnidaria consists of corals, jellies, and hydras. This phylum is characterized by its “gastrovascular cavity with a single opening that serves as a mouth and an anus” (Campbell, 2005, p. 639). Cnidarians have simple nervous systems and it was probably within this group that nervous systems first evolved (Breidbach, 1995, p. 7). The basic plan of the cnidarians nervous system is that of a nerve net that extends throughout the entire body. “An impulse set up in one part of the body goes in all directions more or less equally” (Puranik, 2007, p. 126). “At some locations this nerve net has condensed to form longitudinal or circular nerve tracts” (Breidbach, 1995, p. 8). “One such example of a longitudinal nerve tract is the “giant axon” in the stem of physonectid siphonophores. This nerve tract consists of fused neurons and enables, by virtue of its enlarged diameter, fast signal transduction and, consequently, a fast escape reaction” (Breidbach, 1995, p. 8). Hydrozoan medusae contain the circular condensations in their inner and outer nerve rings at the bell margin. These rings consist of electrically coupled neurons, and “are capable of integrating a variety of sensory inputs and of transmitting signals rapidly throughout the margin” (Breidbach, 1995, p. 8). In Cnidarians, the role of epithelial cells is similar to that of nerve cells, they are able to “receive stimuli and to conduct electric potential in a passive way” (Floreano, 1999, p. 237). Cnidarians also possess muscular cells that serve as the stimulus threshold (Floreano, 1999, p. 237). Other than their basic nervous system, many hydrozoan, cubozoan, and scyphozoan medusa have developed a basic sensory system as well (Breidbach, 1995, p. 8). They have developed light-sensitive organs called ocelli. The light-sensitive neurons in the ocelli “are connected by two or more nerve tracts to the outer nerve ring of the bell margin where the signals are integrated and transmitted to the motor neurons of the inner nerve ring” (Breidbach, 1995, p. 9). Other sensory organs include statocysts, pits that are filled with crystalline material “surrounded by neurons that are connected to the nerve net of the bell” (Breidbach, 1995, p. 9). There are two types of neurons that have been identified in Hydras, sensory cells and ganglion cells. It has also been discovered that in cnidarians there is no difference between these two types of cells. They both “contain a cilium, store dense-cored vesicles at non-synaptic regions, and form chemical synapses with both epithelio-muscular cells and neighboring neurons” (Breidbach, 1995, p. 9). However, not all cnidarians possess these multifunctional neurons. Some neurons only have one or two of the above mentioned features and are only “sensory, motor, or interneurons” (Breidbach, 1995, p. 9). [pic] Phylum Ctenophora, also known as “comb jellies,” is a small phylum of marine animals with 8-comb-like plates for locomotion (Tyagi, 2008, p. 14). The nervous system of this phylum is “diffused type” (Bhamrah, 2001,192). Ctenophores do not have a brain, they have “a sub-epidermal nerve net, meaning they have a network or nerves running beneath the outer skin” (Ramel). These nerves are highly concentrated beneath the costa and at the bases of the 8 tentacles. The “aboral end bears a sensory organ” known as the statocyst (Tyagi, 2008, p. 14). “This organ detects and keeps track of the animal’s vertical position, allowing it to remain upright” (Ramel). There are specific ctenophores that are known to have some “chemical sense organs around their mouth;” one such species is the Nuda, which uses chemicals to detect “the presence of other ctenophores in the water around them” (Ramel). [pic] Phylum Platyhelminthes are flatworms that have bilateral symmetry and the first central nervous system “that processes information from eyes and other sensory structures” (Campbell, 2005, p. 639). The CNS consists of an “anterior concentration of nerve cells – “the brain” – and one or several pairs of longitudinal nerve cords” (Breidbach, 1995, p. 25). The development of the brain led to the growth of a centralized nervous system with “a cephalic ganglion to co-ordinate the input from the frontal sensory receptors and the activity of the two sides of the body” (Breidbach, 1995, p. 27). Flat worms’ nervous system is thought to have a ladder-like configuration of cords connected by commissures, called “the orthogon” (Breidbach, 1995, p. 25). This phylum also has a peripheral nervous system which “consists of a meshwork of nerve fibres and bi- or multipolar nerve cells forming nerve plexuses interconnected with the CNS” (Breidbach, 1995, p. 25). The cephalic ganglion mentioned earlier is also the “center for inhibiting and controlling reflexes in the peripheral nerve net (Breidbach, 1995, p. 27). The amount and structure of plexuses differs, there can be infra- or subepithelial in lower flatworms, and submuscular and specialized plexuses in high flatworms (Breidbach, 1995, p. 25). [pic] The formation of the flatworm brain is and probably will always remain a mystery. “It may have developed from commissures in connection with the nerve net, in connection with the anterior parts of the longitudinal cords, or as an independent structure” (Breidbach, 1995, p. 29-30). Phylum Rotifera are microscopic in size, but are very complex in nature (Campbell, 2007, p. 639). Rotifera have a nervous system that “includes a bi-lobed ganglion and comples sense-organs, tactile, visual, and chemoreceptory arranges in pairs around the body” (Schmid, 2005). It has been agreed that Rotifera’s nervous system “presents itself in the form of a pair of cephalic ganglia, from which proceed nervous filaments” (Knight, 1867, 626). Located “posterior to the corona, the dorsal surface bears, in bdelloids, a long median sensory dorsal antenna” (Schmid, 2005). Near the anterior region, there are tactile and optic sense organs (Schmid, 2005).
[pic]

Conclusion The sensory and nervous systems have greatly evolved through the animal kingdom. From no sensory or nervous system at all to all the complexities of the Chordata, it is amazing to see the natural progression of evolution.

Works Cited
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