how can flatworms and roundworms do function when they dont have organs

For Flatworms:

1.   Respiration	Free-living forms respire through the body surface. Parasitic flatworms are anaerobes.
2.   Circulation	The circulatory system is absent.
3.   Digestive Tract	The digestive tract is incomplete, i.e. without an anus. However, a muscular suctorial pharynx and highly branched intestine are present.
4.   Excretion	They are ammonotelic. They possess protonephridia which contain flame cells.
5.   Skeleton	They do not have any skeleton. Some flatworms have a cuticle which protects their body from the host’s digestive juices.
6.   Nervous System	It consists of the brains and two longitudinal nerves. Both nerves are connected at intervals by transverse commissures. Hence, the nervous system appears ladder-like.
7.   Reproduction	Asexual reproduction is by transverse binary fission. Possess highly developed reproductive organs. All flatworms are hermaphrodites, except Planaria which is a unisexual animal.

 

For Roundworms:

1.   Respiration	A respiratory system is absent.
2.   Circulation	A circulatory system is absent.
3.   Digestive Tract	The digestive tract is complete. Shows the presence of the muscular pharynx.
4.   Nutrition	Parasitic and found on plants and in animals.
5.   Excretion	Excretion is carried out by a large H-shaped renette cell.
6.   Skeleton	Skeleton is absent.
7.   Nervous System	A circumpharyngeal ring is present from which nerves arise forward and backward.
8.   Reproduction	Sexes are separate. Show sexual dimorphism. The male is smaller than the female, and its posterior end is curved. Asexual reproduction is absent.