Phylum Annelida Digestive & Respiratory Systems

Instructor: Jeremy Battista
Animals of the world are broken down by similar characteristics. One of the first levels after kingdom, is phylum. The Phylum Annelida covers the worms, such as earthworms. What makes them special? We look at their digestive and respiratory abilities here.

What Is Phylum Annelida?

The phylum called Annelida can be summed up in one word: worms. Annelids are those animals known as the segmented or ringed worms. This includes but is not limited to earthworms and leeches. This differ from another phylum of worms, platyhelminthes in that they are much more complex than their flatworm brethren.

Notice the small ridges that separate the different segments of the worm.
Lake Striped Worm

Characteristics

As the above description suggests, the creatures in this phylum are segmented worms. If you have ever seen an earthworm, you know what I am talking about. There are small rings that are connected to form the worm. They, like most simplistic animals, are bilaterally symmetrical (the left is just like the right), and they have a body cavity or coelom.

Each of the segments that makes up their bodies has almost identical organs as the segment before it with the exception of the 'head' and the 'end' of the animal. These ends are technically not segments because they contain different organs than all the other segments.

Digestion

Since an annelid's segments contain the same organs as all the others, there is little need for a very specialized digestion system. Instead, the digesting occurs through a long tube that extends from the mouth to the anus. It is held in the center of the worm as one continuous structure. The tube itself is unsegmented unlike the worm's outside structure. Basically you can think of a worm's digestive tract as similar in scope to a human's or, maybe even better, a bird's digestive system.

If we were to zoom in to the digestive tract of something like an earthworm, we would see some similarities to us as well as birds. Clearly worms do not have teeth, like we do, so they need a structure to mash up the food. This is found in the gizzard, like a bird. The worm takes in the food through it's mouth and pharynx and passes it down its esophagus. It gets stored in the crop, similar to our stomach, before it gets smashed up in the gizzard, the final stop before the intestine.

Just like a human, the intestine contains chemicals that breakdown the food for the worm, absorbing the nutrients through the intestine wall. Once it takes what it needs from the food, the rest is excreted through the anus, again, similar to birds.

Respiration

Simply put, respiration occurs through annelid skin. Underneath the actual skin of the animal exists capillaries or very tiny blood vessels. In the capillaries is, you guessed it, blood! The blood allows oxygen to absorb through the skin into the capillaries, and it gets shuttled around the worm via the oxygenated blood.

As the worm comes into contact with oxygen, the oxygen will absorb directly through the worm's skin. This is done through the process of diffusion. Recall from high school biology that diffusion is where molecules, in this case oxygen, move from an area of high concentration to one of low concentration. So, outside of the worm there exists our atmosphere of around 21% oxygen. As the worm uses up its available oxygen, the amount of oxygen inside of the worm is lower and in less concentration than outside, so the oxygen outside will absorb into the worm.

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