The Earthworm: A Simple Protostome

This week, we will be dissecting earthworms (Lumbricus terrestris) to explore the properties of the smooth muscle that controls the movement of food in the digestive system. Smooth muscle is formed from one of the three primary types of tissue that originate early in the development of an embryo. It is not under voluntary control and responds to various hormones and neurotransmitters.

In order to understand the structural complexity of an organism, it is helpful to know something about the manner in which it develops. Earthworms are hermaphrodites and contain both male and female reproductive organs. After mating, eggs are laid in cocoons where fertilization occurs. After fertilization, the egg begins dividing. By the time it has reached the four celled-stage, the cellular material has been partitioned into different daughter cells. The fate of these cells has been determined and they would not be capable of developing into a complete organism if allowed to develop independently. This is different from frog embryos where, at the four-celled stage, each cell could develop into a complete frog (this cellular capability is termed totipotency).

The worm embryonic cells continue to divide forming a hollow ball or blastula. One side of the ball flattens and folds to the inside creating a pocket. The cells that line this pocket are endoderm and will give rise to the digestive tract. The cells that remain on the surface are ectoderm and will give rise to the skin and nervous system. The third layer of the embryo, the mesoderm, is derived from the cells that migrate into the interior space between the endoderm and ectoderm. These cells divide, forming a solid band of cells. The mesodermal band splits, creating spaces known as the coelom, or body cavity. The blastopore, the opening through which the cells migrated to form the three-layered embryo, will eventually become the mouth. This is why earthworms are called protostomes. The word is derived from Greek and means "first mouth."

The relationship between the embryonic cell layers can be more clearly understood by examining a longitudinal-section. Mesoderm gives rise to muscle and a variety of tissue types. The mesoderm adjacent to the body wall differentiates into a lining known as the parietal peritoneum and the mesoderm covering the digestive tract differentiates into visceral peritoneum. Also note that the mesoderm has also given rise to dorsal and ventral vessels. At the anterior end, these vessels are connected by pulsating structures called hearts. The mesodermal tissue also contributes to the septa that separate the segments. (A sense of how these septa form can be gotten by viewing the early embryo again.) The digestive organs pass through these septa and are bathed in the fluid that fills the coelomic spaces. Near the posterior end of the adult model, the mesodermal layer has been removed so the intestine is visible. The ventral nerve cord derived from ectodemal tissue also passes through the septa.

A cross-section through the worm further illustrates these relationships. The exterior surface of the worm is covered by a thin cuticle that is secreted by the underlying epidermis. The circular and longitudinal muscles that power locomotion and movement lie just under the epidermis. The coelom is lined with peritoneum with the digestive tract suspended in the coelom. Note the folding of the dorsal wall of the digestive tract; this effectively increases the surface area without increasing the diameter of the tube. Between the inner and outer walls of the intestine is a layer of smooth muscle. Adjacent to the smooth muscle is a layer of chloragogue cells that are derived from the mesoderm and are thought to be associated with digestion. As you examine the cross-sectional slide of an earthworm, can you find the layers labeled on the model?

During the laboratory, you will be isolating a portion of the digestive system and exploring the function of the associated smooth muscle. The region of interest contains the crop and the gizzard. These structures lie posterior to the esophagus, which can be easily identified because it is encircled by enlarged blood vessels that function as hearts. The crop-gizzard is also partially obscured by the cream-colored seminal vesicles of the reproductive system.

Since you will begin your dissection with an incision along the dorsal surface, it is important to familiarize yourself with the basic external anatomy. You will need to be able to identify the following features when you begin your dissection.