Unsegmented Worms:Flatworms and Roundworms

Biology 11

Worms

• Divided into 2 Groups

–Unsegmented• Flatworms• Roundworms

–Segmented• Annelids

Phylum Platyhelminthes

• Platy= flat• Helminth=worm

• The Flatworms

Characteristics

• Acoelomates (no fluid filled space)• Unsegmented body (not divided into sections)• Live in aquatic habitat (streams, lake, oceans)• Free living or parasitic• Cephalization- concentration of sensory

organs at one end• Bilaterally symmetrical

Germ Layer

• 3 cell layers- ectoderm, mesoderm, endoderm• Have a two way digestive system (lack an

anus)

Class Tubellaria

• Free living flatworms• Marine and fresh water (a few terrestrial)• Reproduce asexually by fission and sexually

are hermaphrodites

Class trematoda

• Parasitic flukes• Live as external and internal parasites on fish

and other animals• Complex parasitic life cycle involving hosts• Hermaphrodites

Class Cestoda• Parasitic tape worms• Live in intestines of vertebrates• Have scolex in “head” section that have sections

called proglottids– Contain reproductive organs and fertilization takes

place inside– Mature proglottids detach and pass out of the

intestine

Phylum Nematoda

• The roundworms• Found in soil, marine, and fresh water

• Cylindrically shaped• Bilaterally symmetrical• 3 body layers- have pseudocoelom

• One way digestive system – Have mouth and anus

• No circulatory or respiratory system– Fluid in pseudocoelom distributes food and

oxygen• Have nerve ring around pharynx (no brain)• Have separate sexes

E X T R E M E

H E E B Y – J E E B I E S

A H E A D !

Roundworms found in dog poop

Inside a chicken’s intestine

Feeding

• Most are free living carnivores/omnivores• Have pharynx• Can be parasites in plants and animals

Respiration and Circulation

• Diffusion of gases through body walls

Excretion

• Metabolic wastes diffuse through body walls

• Undigested wastes through anus

• Filarial worms• Tropical Asia• Mosquito carries worms

from host to host• Cause elephantiasis