Homosexuality and Religion

Invertebrates: Planaria

I. Abstract:

The purpose of this experiment is to study the movements and regeneration abilities of the planarian flatworms. Planaria reproduce by means of regeneration, which can be considered a form of asexual reproduction. A severed part of an organism grows into a new individual and the parent organism regenerates the missing part. Using a small pipette, we obtained a planaria from a culture sample and placed it on a clean slide. Using a razor blade, we cut off the head of the planaria and split the body into two equal parts. After the first week, we noticed that the planaria had begun, but not completely, regenerating its body parts. After the second week, the planaria had died. One piece of the planaria had shriveled up and decreased in size. The other half of the planaria body had disappeared. If the planaria had lived throughout our experiment, we believe that each body part would have fully regenerated, the head growing at the posterior end and the tail growing at the anterior end, and form two sets of planaria.

These flatworms dwell under rocks, submerged leaves, and other debris, and feed on aquatic insects, microcrustaceans, and proteinaceous detritus as scavengers, using a retractable pharynx (Kelly, 2004). They secrete enzymes from the mouth that partially digest the prey while it is pinned by the pharynx. Once the food is softened, the mouth sucks in the food and digestion is completed. Since the planarians are negatively phototropic, the eyespots do not actually form images, but they detect light. Planarians have primitive brains and a complex central nervous system. They can also be taught simple tasks such as traveling through a maze to feed, or to respond to other stimuli.

II. Introduction:

Planarians display bilateral symmetry, meaning they have distinct left and right sides, and organ level of organization (Shaw1998). Regeneration begins by the formation of a blastema which is a small blob of clear cells at the cut surface. The blastema is made up of undifferentiated cells which arise from special cells in the parenchyma, called neoblasts. Neoblasts are RNA rich cells which are undifferentiated and are set aside early in the embryology of the planaria. The blastema and newly generated parts of planarians are generally characterized by lack of pigmentation and are readily distinguished from the older body parts. Regeneration in planarians takes place under natural conditions as well as artificially in the laboratory. Under natural conditions the planarian ordinarily fragments into two parts, with the line of fission occurring just posterior to the pharynx. The anterior part then regenerates a new tail while the posterior fragment develops a new head.

Planarians are members of the genus Dugesia, which fall under the Class Turbellaria, in the phylum Platyhelminthes and under the kingdom Animalia (Campbell, 2004). They are largely free-living, freshwater invertebrates found in or near the sediments of ponds and streams. Planarians are carnivores that prey on smaller animals or feed on dead animals. Free-living flatworms do not have specialized organs for gas exchange or circulation. The cells are placed close to surrounding water because of the flat shape of the body and food is dispersed via fine branches of the gastrovascular cavity. Nitrogenous waste in the form of ammonia diffuses directly from the cells into the surrounding water. Flatworms also have a relatively simple excretory apparatus that functions mainly to maintain osmotic balance between the animal and its surroundings (Campbell, 2002). They use flame cells that have open ducts to the outside environment in order to maintain an osmotic balance. Planarians have triangular heads, flat worm-like bodies and can respond, and even modify their responses, to stimuli such as heat, and have eyespots to sense light and lateral flaps for smell reception. They move by using cilia on the ventral epidermis, gliding along a film of mucus they secrete. Planarians can regenerate if sliced up and they sometimes use this as a means of asexual reproduction. The parent constricts in the middle, and each half regenerates the missing end. Planarians are hermaphroditic, however they copulate for cross-fertilization in sexual reproduction (Campbell, 2002). In this experiment, we believe that our planaria, using its regenerative abilities, will grow back its bisected parts.

Campbell: Anatomy of a planarian

III. Methods

We began our experiment by using a small pipette to obtain our planaria from the cultured sample and placed it on a clean slide (Geiger, 2004). Since the planaria is exothermic, meaning they release heat, we used an ice cube to slow them down and make our cuts. Using a razor blade and working under a dissecting microscope, we decided to follow the first experiment in the procedures and cut the planaria into three parts; we first cut off part of the head just below the eyes and discarded it. Then we made a second cut longitudinally along the length of the body. We put the body parts into a petri dish and filled it with water. We placed the petri dish on the shelf in the microscope cabinet where it was cool and dark. Each week we changed and made sure the planaria had an adequate amount of water. We observed the regeneration levels of the planaria and recorded observations for our data.

IV. Results:

After the first week of leaving our planaria in the shelf of the microscope cabinet, we found the planaria used its regenerative abilities and began growing back its missing parts. At first the planaria shriveled up, most likely because of its response to the light. It also