The Worms

Phylum Platyhelminthes (flatworms):

General Characteristics of flatworms:

  • These are the first organisms to exhibit three distinct cell layers. The ectoderm, mesoderm , and endoderm.

  • They are classified as acoleomates meaning they have no true body cavity (coelom).
  • They contain bilateral symmetry. These organisms exhibit cephalization. Many of there sense organs are grouped at the anterior end of the organism. This affords them directional movement. Usually the first part of an organism that enters a new environment is the head.

  • They contain rudimentary organ systems such as: a ladder-like nervous system with a small brain and in some cases photosensitive eye spots. A network of flame cells used to sweep nitrogenous waste out of the body. A primitive digestive system consisting of a mouth, pharynx and branched gastrovascular cavity.
  • Flatworms exhibit two forms of life styles, free-living and parasitic.
  • The phylum contain 3 classes: Trematoda (parasitic flukes), Cestoda (parasitic tapeworms ), and Turbellaria (free living Planaria ).
  • They can reproduce asexually (regeneration) or sexually (hermaphroditic).
  • The parasites exhibit a complex life cycle usually involving several intermediate hosts.


The Tapeworm:

Tapeworms are intestinal parasites that usually infest mammals. They are rather long flat animals sometimes growing to lengths of 40 feet. The animal is composed of several parts: The head area is called the scolex. The scolex contains a ring of hooks at its tip and is ringed with lateral suckers. (Locate these parts on the picture below). The neck is where the proglottids or body parts grow from. The animal does not contain a mouth nor digestive organs, since it lives off of the host's digested food. The animal self fertilizes and sends it eggs out of the host along with the fecal material. Humans usually get infected with tapeworm by eating undercooked infected beef.



The Fluke: The fluke is an internal parasite usually living in the host's liver. There are several species of fluke, the sheep liver fluke and Chinese liver fluke. Flukes must spend their intermediate development in special types of snails. If the snail is not present the life cycle of the fluke will end. Humans pick up flukes in several ways: Eating infected fish and walking barefooted in infected waters. Flukes contain 2 suckers on its body, one ventral and the other anterior. They have a well developed digestive system and reproduce hermaphroditically. Their eggs leave the body with the solid waste of the host. 

The Planarian: The planarian is a free living flatworm. It is found in slow streams and ponds near the shore. They do not like light so they hide under the decaying vegetation. They are very fond of liver and will come swimming if some is placed where they live. They contain a remarkable power to regenerate their lost body parts. And can also be trained to do simple tricks. They are hermaphroditic as well. They contain two eye spots that give the worm an appearance of being cross eyed.



Phylum Nematoda : (Roundworms) Ascaris, pin worms, hookworms, and Trichina

General Characteristics:

  • They contain three cell layers.


  • They possess bilateral symmetry.
  • They are pseudocoelomates. Their body cavity is partially lined with mesoderm so they call it a false body cavity.
  • They possess a one way digestive system. Mouth, intestine and anus.


  • They contain separate sexes. The female being the larger of the pair. Females are capable of producing 100 of thousands of eggs per day. The male usually dies after copulation.
  • Many of the phylum are free living worms but some are parasitic and cause many terrible diseases all throughout the world.
  • They have a circular brain and several longitudinal nerve cords. The nitrogenous waste is collected by two lateral canals and emptied from the body through an excretory pore located in the head area.
  • They contain longitudinal muscles only. This makes it difficult for the worm to move in a coherent manner.
  • Their body is covered with a tough cuticle. This prevents the parasitic worms from being digested and the free living worms from drying out.
  • Parasitic worms exhibit a complex life cycle but much of the cycle is shared by most of the parasites. The mode of entry into the host vary from injection by an insect, to drinking egg fill contaminated water. Once the eggs or larvae make it into the body the organisms take a 10 day trip around the body growing and developing. After the trip is over most of them come out in the lungs and crawl up to the throat, where they are reswallowed into the intestine where they take up residence. Worms like the Trichina worm enter the muscle tissue and remain their until someone eats them. Filarial worms enter the lymph canals and block them causing massive swelling.