Parasitic helminths have the following characteristics, except are multicellular animals have a definitive host where the adult form lives have developmental forms that include cysts include roundworms include tapeworms.

The only characteristic Parasitic Helminths don't have is:

have developmental forms that include cysts

Explanation:

First of all to understand this answer we need to analyze several things. First, the cyst is a parasitic form adopted to increase resistance to environmental conditions. While the eggs are not parasitic states of the living being. Instead, they are stages in which the living being develops without the need of practicing parasitation. Second, it is not made to resist environmental conditions, but to develop until the living being is strong enough to come out of the egg and live by itself. Now, helminths don't have cysts because their species have different reproductive and developing forms ...

Helminths are metazoans, multicellular animals, not vertebrates, without articulated appendages and with a musculoskeletal envelope that surrounds the general cavity or celoma.

Explanation:

Under the cuticle they have the muscular layer, responsible for the movement. The digestive system is a long tube with two openings, a mouth and an anus. They lack circulatory system and respiratory organs. The nervous system is formed by a pair of ganglia from which the nerve branches that are arranged longitudinally. They have developed sexual organs. The cestodes and most of the trematodes are hermaphrodites.

The helminths of interest in human pathology are divided into two Phylum:

-Nematoda: cylindrical worms, not segmented and with separate sexes.

-Platyhelminthes: flat worms, can be segmented and also hermaphroditic.