Planaria are nonparasitic flatworms (which make great pets) of the biological family Planariidae, belonging to the order Seriata. Planaria are common to many parts of the world, living in both saltwater and freshwater ponds and rivers. Some are terrestrial and are found on plants in humid areas. These animals move by beating cilia on the ventral dermis, allowing them to glide along on a film of mucus. Some move by undulations of the whole body by the contractions of muscles built into the body wall.
They exhibit an extraordinary ability to regenerate lost body parts. For example, a planarian split lengthwise or crosswise will regenerate into two separate individuals. The size ranges from 3 to 12 mm, and the body has two eye-spots (also known as ocelli) that can detect the intensity of light. The eye-spots act as photoreceptors and are used to move away from light sources. Planaria have three germ layers (ectoderm, mesoderm, and endoderm), and are acoelomate (i.e. they have a solid body with no body cavity). They have a single-opening digestive tract, consisting of one anterior branch and two posterior branches in freshwater planarians. Because of this three-branched organization, freshwater flatworms are often referred to as triclad planarians.
The most frequently used in the high school and first-year college laboratories is the brownish Dugesia tigrina. Other common varieties are the blackish Planaria maculata and Dugesia dorotocephala. Recently, however, the species Schmidtea mediterranea has emerged as the species of choice for modern molecular biological and genomic research due to its diploid chromosomes and existence in both asexual and sexual strains. Recent genetic screens utilizing double-stranded RNA technology have uncovered 240 genes that affect regeneration in S. mediterranea. Interestingly, many of these genes are found in the human genome.
It should be noted that the term "planaria" is most often used as a common name. It is also the name of a genus within the family Planariidae. Sometimes, it also refers to the genus Dugesia.
Anatomy and physiology
The planarian has very simple organ systems. The digestive system consists of a mouth
, and a structure called a gastrovascular cavity. The mouth is located in the center of the underside of the body. Digestive enzymes secrete from mouth to begin external digestion. The pharynx connects the mouth to the gastrovascular cavity. This structure branches throughout the body allowing nutrients from food to reach all extremities. They eat living or dead small animals that they suck with their muscular mouth. From there, the food passes through the pharynx into the intestines and digesting of the food takes place in the cells lining the intestine, which then diffuses to the rest of the body.
Planare receive oxygen and release carbon dioxide by diffusion
. The excretory system is made of many tubes with many flame cells
and excretory pores on them. Flame cells remove unwanted liquids from the body by passing them through ducts that lead to excretory pores where the waste is released on the dorsal surface of the planarian.
At the head of the planarian there is a ganglion
under the eyespots. From the ganglion there are two nerve
cords which connect at the tail. There are many transverse nerves connected to the nerve cords which make it look like a ladder. With a ladder-like nerve system, it is able to respond in a coordinated manner.
Planaria are hermaphrodites, possessing both testes and ovaries. Thus, they can reproduce asexually
with their own gametes or sexually
with another planarian. In asexual reproduction, the planarian detaches its tail end and each half regrows the lost parts by regeneration
. However several problems can occur with this, so this does not happen very often. Instead, in sexual reproduction, each planarian transports its excretion to the other planarian, giving and receiving sperm
. Eggs develop inside the body and are shed in capsules. Weeks later, the eggs hatch and grow into adults. Sexual reproduction is desirable because it enhances the survival of the species by increasing the level of genetic diversity