In details


People often react with some disgust when they hear the word "worms".

The impression you get is something slimy, creepy and dangerous to health. The word worms, applied to the diseases caused by some of these animals, contributes to this notion. Indeed, some worms are disease-causing, especially in populations living in poor health.

However, there are numerous worms that are totally harmless and non-disgusting in appearance.

Three are the animal phyla that are most often recognized as worms: Platyhelminthes, Nematoda and Annelida.

Phylum Platyhelminthes: Flat Worms

The flatworms are worms with a flat back-ventral body (platy= boring; helminth= worm), with bilateral symmetry (first appears on evolutionary scale).

There are approximately 20,000 described species of flatworms. They may be parasitic or free-living, which may occur in the seas, freshwater or humid terrestrial environments.

As human parasites we can cite the tapeworm and the Schistosoma mansoni, cause of schistosomiasis. Other animals can also be parasitized like ox, pig, dogs, cats, etc. The body may or may not have a segmentation. Most species are monoecious.


Are accelerated (do not have celleloma) and triblastics (have the three germinal leaflets: ectoderm, mesoderm and endoderm). They have bilateral symmetry.

The ectoderm gives rise to the outer lining, the mesoderm gives rise to the musculature and the parenchyma, which is a tissue that fills the entire space between the intestine and the body wall. The endoderm gives rise to the intestine and its lining.


Platelminths have a simple epithelium, and the epidermis is formed by a simple layer of cells. The parasitic species have a protective cuticle and, in some cases, suction cups. Some have eyelashes in the ventral region for locomotion purposes. They may have mucous cells, which produce lubrication to facilitate locomotion.


The digestive system of the flatworm is incomplete, ie the mouth is the only opening to the outside without anus. Digestion may be intra or extracellular. The intestine is quite branched, which facilitates the distribution of digested food. What is not used in digestion is eliminated by mouth. The planarians have the mouth in the ventral region and a prototype pharynx (exteriorized), which facilitates the capture of food by sucking.

At tapeworms have no digestive system, feed by diffusion, absorbing the pre-digested nutrients of the host.


They have no respiratory system, and gas exchange is made by the epidermis by diffusion. This type of breath is called cutaneous or cutaneous and occurs in free-living species, as parasites make anaerobic respiration.


The flatworms do not have circulatory system. Digested food is sent to cells by diffusion, thanks to a well-branched intestine because it is gastrovascular.


They are the first animals to have an excretory system: protonephidium, which is formed by several excretory tubules with flame cells. Flame cells are fundamental in this excretory system. They have several flagella that promote the movement of fluids, making them very well filtered.

Waste falls into a system of ducts or tubules, which open outwards through structures called nephridiophores, which are excretory pores. These pores are located on the dorsal surface of the body laterally.


They have no skeleton.

Nervous system

They feature a process called cephalization, ie a head with nerve and sensory structures. The nervous system of flatworms is called ganglion, It consists of two nerve ganglia, which are connected to two ventral and longitudinal nerve cords, which are connected by transverse commissures and run through the entire ventral region to the back of the worm.

Freshwater planarians have two eyelashes in the head region, photo-receptor structures. These structures are not capable of forming images, only perceiving light.

Nas auriculae, lateral regions of the head, are present chemoreceptor cells, able to perceive various chemicals that are dissolved in the water.


The musculature is of the smooth type, which favors the movement and locomotion of the animal, and may have the collaboration of cilia, if present. This smooth musculature forms the muscle-dermal tubule, which is a functional unit with the skin.