Phylum Aschelminthes

General Characters

• Bilaterally symmetrical, triploblastic, unsegmented, cylindrical worms.
• Organ-system level of organization.
• No appendages in roundworms.
• The body wall consists of firm, non-living, resistant cuticle, epidermis and muscle layer. The cuticle is moulted (changed) four times during growth period. The epidermis is syncytial, but lacks cilia. The musculature contains longitudinal fibres only.
• There is no mineralized skeleton. High fluid pressure in the pseudocoelom maintains body shape. It is called hydroskeleton.
• There is a straight, one way digestive tract with mouth as well as anus. Such a digestive tract is said to be complete. Mouth is bordered with 3-6 lips having sensory papillae. Pharynx is muscular with 3-rayed cavity. It is used to suck the food. Intestine is non-muscular.
• Respiration occurs by diffusion through the body surface.
• The circulatory system is undeveloped.
• Nervous system consists of circumpharyngeal nerve ring & six longitudinal nerve cords.
• Excretory system consists of gland cells or canals or both. Some forms have protonephridia.
• They exhibit sexual dimorphism; males being smaller than females. Fertilization is internal. Development direct. There is no asexual reproduction.
• Cleavage is determinate and spiral.

Nematoda Characters :

• The Nematodes are commonly called roundworms or threadworms.
• Body is cylindrical, tapering at one or both ends.
• Cilia are lacking.
• Mouth often has lips with sensory papillae. Pharynx is muscular with 3-rayed cavity. Intestine is non-muscular.
• Male genital tract joins the digestive tract to form cloaca. Female has a separate genital aperture.
• There is sexual dimorphism, males being smaller than the females.
• Development is direct.
• The roundworms perform peculiar twisting movements.
• They may be free-living or parasitic in animals or plants.

Fig. Nematodes : A. Enterobium (female),B. Ancylostoma (male)