This is because the earthworm takes in oxygen through its skin and releases CO2 through it's skin as well using diffusion
Because if it all stays in then your body will keep in this poisons gases. http://wiki.answers.com/Why_does_air_move_in_our_out_of_your_respiratory_system#ixzz1ZCdvdlbk
Blood, cardio vascular, Respiratory, skeletal and your digestive system are all essential counterparts to the muscular system without these other systems the muscular system would not be able to function, its important to remember that the muscular system need oxygen, nutrients, hormones, gases and proteins and calcium to function and build.
ventilation
the lungs and respiratory system
Diffusion
Material moves from the respiratory system to the circulatory system by diffusing across a membrane in the air sacks called alveoli.
In the alveoli
by diffusion across a capillary wall
Grasshoppers as well as other insects have a tracheal system instead of a respiratory system to move gases. A respiratory system requires energy to move the gases where as the tracheal system uses a process of simple diffusion which does not use any energy.
Three (of many) are: * the muscular system and the skeletal system. All muscles move bones. * the circulatory system and the respiratory system meet in the lungs for the exchange of gases * the circulatory system and the urinary system meet in the kidneys where wastes are removed
diffusion
the brain is essential for the movement of the earthworm. If the brain of the earthworm is removed, the earthworm will move continuously without stop