Organism Physiology The Planarian or "cross-eyed flatworm" is a relatively simple invertebrate organism, belonging to the animal kingdom and the phylum of Platyhelminthes which covers all flatworms. The phylum of Platyhelminthes contains four separate classes: three of these cover parasitic forms of flatworm that live inside other organisms (like the famous tapeworm). The fourth class, Turbellaria, includes all non-parasitic flatworms. One of these is the Planarian, which exists in a number of different genus and species: our diagram is derived from the standard Planarian used for biology students, Girardia Tigrina. A close examination of the structures and functions of the main organs found in this simple organism can explain how the Planarian has evolved physiologically to become suited to its environment.

The first organ to consider in the Planarian is not an internal one. The epidermis, or outer skin layer, of the Planarian (numbered 4 on the diagram above) is a perfect example of adaptation to environmental conditions. First off, we may note that internally the Planarian contains no respiratory or circulatory system: these functions are covered by the epidermis, which absorbs oxygen from the surrounding water, and diffuses carbon dioxide outward in the same way. This Planarian is a completely aquatic animal, therefore the outer skin layer has what Littlewood and Bray (2001) describe as "a complete and dense covering...

This is the Planarian's chief means of movement in an aquatic environment. We can see on the diagram that this is a flatworm with no limbs of any kind, so how does the Planarian swim in an aquatic environment? The answer is through its system of movement and its external epidermis and cilia. Obviously although the Planarian is basically flat (hence the name "flatworm) we can see that nevertheless has numerous different layers comprising its body, indicated as 3 and 5 on the diagram, the transverse muscle layer and the longitudinal muscle layer. These terms merely indicate the directions in which the muscles are facing (side to side, or front to back) but they obviously can also give us a sense of how the Planarian is able to move. Muscular contractions in the two muscle layers give the worm a certain ability to move, and to expand or contract its small flat body, but the cilia on the epidermis basically act like little oars, and allow these muscular movements to turn into forward motion for the Planarian.
If we were to see a Planarian swimming like this in the water, perhaps while feeding, we would notice its single largest external feature, the pharynx (indicated on the diagram as 8). The pharynx has a dual function: it serves the Planarian as both mouth and anus. Seen from the side, the pharynx is actually quite long and can be…