Earthworm dissection anatomy and observations

Earthworm Dissection

Phylum Annelida is comprised of a group of animals referred to as segmented worms. They are can be found universally in terrestrial, marine and fresh water environments. They can range from a few millimeters to several meters in length. Annelids can be differentiated from other invertebrates based on their external structure. These include lengthy cylindrical bodies composed of several segments of the same kind, lack of appendages, antennas or prominent head part. There are 3 major Annelid groups -- Oligochaeta, Hirudinidea and Polychaeta. The first two are terrestrial and associated with earthworms and leaches respectively. The last one is marine in nature ("Annelida: Worms," n.d.)

Earthworms have front and back ends, better acknowledged as the anterior and posterior. The first posterior section is called its mouth which has a cushioning of flesh called prostomium. This is used to block the mouth entrance when the anterior contracts. While hunting for food, the prostomium extends out and ingests particles ("Composting Worms," n.d.).

The anterior cross section of the gizzard of an earthworm consists of the oral cavity part of its digestive system. This includes the mouth and the Pharynx. The mouth is used to ingest soil and serves as a way of locomotion. This is assisted by the pharynx where the pharyngeal muscles are utilized in the sucking action. Those substances are then passed onto the oesophagus. The digestive tube has a nerve cord underneath it which broadens into a large cerebral ganglion ("Earthworm," n.d.).

The earthworm does not have a heart per se. However they have organs called aortic arches which show functional and structural similarity with the human heart. It has 5 of these and can be related to the 4 chambers in the human heart. A gland in between each arch facilitates calcium digestion. The arches and gland put together makes up less than 5% of their total body length. They are found enfolded around the oesophagus, behind the ganglion. An earthworm needs all 5 of their arches to support its blood flow, each of which work in the same manner (Sherwood, 2010).

Earthworms are hermaphrodites since they can generate both sperm and eggs. Mating involves 2 earthworms inseminating each other where sperms are exchanged. The male and female sexual cavities are located near the clitellum. They connect the lower ends of their anteriors with heads positioned in reverse directions. Mucous secreted joins the openings with slime. Sperm is pushed into the longitudinal channels formed by the mucous. They reach the sperm receptacles of their partner, which then break apart. Laying of the eggs and fertilization follows ("Native Australian," 2006)