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Bullatoverms (Wrinkle Worms)

Bullatoverms are worms with a coelom, closed circulatory system and true segmentation. The coelom is generally divided by walls called septa. In the most forms the compartment corresponds to a segment of the body, which also includes a portion of the nervous and circulatory system. The closed circulatory system consists of networks of vessels containing blood with oxygen-carrying hemoglobin, with dorsal and ventral vessels connected segmental pairs of vessels. The dorsal vessels are muscular and pump blood through the circulatory system. Tiny blood vessels in the skin function as a respiratory organ. The nervous system consists of a nerve cord from which lateral nerves come in contact with each segment. Every segment is autonomous, however they unite to perform for single body functions such as locomotion. The vascular system and the nervous system are separate from the digestive tract. The vascular system includes a dorsal vessel conveying the blood toward the front of the worm, and a ventral longitudinal vessel which conveys the blood in the opposite direction. The two systems are connected by a vascular sinus and by lateral vessels of various kinds, including capillaries on the body wall. Reproduction is both sexually and asexually.

The epidermis secretes a layered cuticle made of collage that protects the body. Although this cuticle allows movement and shape changes via a hydrostatic skeletal system, it is very inelastic and not allow the volume of the worm to increase. Therefore, the worm has to molt and form new cuticles as it grows. However, there are large gaps between the cutuicle layers whiche give the wrinkle worms their name.

Some wrinkle worms secrete calcareous tubes.