Earthworm Dissection

After learning about Annelids in class, we all had a chance to dissect a earthworm. While it sounds like no big deal, I was pretty close to panicking; I am completely terrified of anything that creeps, crawls or slithers. To add to that, we were told that no gloves would be provided so we had to touch the worm with our hands. I almost died. Well, not really, but I wasn't too keen on the touching part, no matter how interested I was in the worm's anatomy. In the textbook, we saw many simplified diagrams of worms. So, naturally, this lab was aimed to let us find and observe these anatomies in a real specimen. It was a simple "look at the diagrams and identify the parts in the worm" kind of lab. However, this proved to be more challenging than I expected. I don't think any of us realized how simplified the diagrams were until we started dissecting the worm. The pictures in the textbook barely resembled the real anatomical structures. It was very tough to figure out what structure was what. In fact, some organs weren't even in the correct (standard) position due to our scalpels messing everything inside up. Also, the diagrams in the textbook were 2-D so we could see all the organs clearly. However, in the real specimen, the organs were layered one over one another and intertwined with others. This lab proved to me that while these worms may be considered primitive and are simple-looking, their appearances were deceiving as their anatomies were quite complex. It made me wonder just how complex and puzzling human bodies must be.

In this picture, we see the worm's five hearts which are also known as "aortic arches." They pump the blood through the worm's closed circulatory system. 

The digestive system starts at the mouth where the food enters. The pharynx pumps the food through the esophagus, through the crop (where it may be stored, to the gizzard (where the food is ground up). It then passes through the intestine and the waste is eliminated through the anus. 

Brain blocked by pin. Standard area of brain pointed out. 

The worm's "brain" is made up of paired ganglia (cluster of nerve cells) which are connected to the rest of the body by the ventral nerve cord. 

The nephridia deals with waste resulting from cellular metabolism. They remove waste products from body fluids and carry them to the outside. Solid waste is passed through the anus which is connected to the digestive system. 

We can assume the earthworms eat soil because of the large concentration of soil in the earthworm's digestive tract. If we wanted to test this theory out,  we can simply place a worm in an environment that only contains soil. If the worm survives, we can conclude that it eats soil. Or we can simply google what earthworms eat. 

Setae are tiny bristle-like hairs on the earthworm that help it anchor itself and move through the soil.

Photo Credit: http://images.tutorvista.com/content/animal-nutrition/digestive-tract-earthworm.jpeg

The small gizzard grinds up the relatively small amounts of food. The large amount of undigested ingested soil passes through the long intestinal tube. The difference in proportion between the gizzard and the intestine indicates the adaptation of the earthworm's digestive system.

While my dissection of the earthworm did not go past segment 32, I expect to see more of the intestine and the undigested food if I were to dissect the remainder of the worm/

Earthworms are hermaphroditic so they possess both male and female reproductive organs. The ovaries produce eggs and the testes produce sperm. However, worms do not self fertilize so it must join with another worm to reproduce. They connect at the clitellum and the sperm is released through the male genital pores, along the sperm grooves to the female genital pores to internally fertilize the eggs of the other worm. The clitellum then secretes a mucus ring into which the eggs and sperms are released. Then the ring slips off the worms and develops into a cocoon that shelters the fertilized eggs. 

While this lab was kinda icky in my opinion, I enjoyed the dissection immensely. I felt like a surgeon from a hospital drama. Through this lab, not only did we learn about the anatomical features of an earthworm, we also learned that creatures are more complex than textbooks make them out to be.