of a living organism to emit light) is well marked in ctenophores. They lack nematocysts but possess special cells called lasso cells or colloblasts which help in food capture. Digestion is both extracellular and intracellular. Sexes are not separate (monoecious).
They reproduce only by sexual means. Fertilization is external and development is indirect and includes a larval stage called cydippid larva . e.g., Pleurobrachia (Figure . ).
Examples : Pleurobrachia and Ctenoplana . Figure . Example of Ctenophora- Pleurobrachia . .
Phylum: Platyhelminthes (Flatworms) (G. Platy -broad or flat; helmin -worm) They have a dorsoventrally flattened body and hence called flatworms. These animals are bilaterally symmetrical, triploblastic, acoelomate with organ system level of organisation. They show moderate cephalization and unidirectional movement.
They are, mostly endoparasites of animals including human beings. Hooks and suckers are present in the parasitic forms and serve as organs of attachment. Their body is not segmented, but some exhibit pseudosegmentation. Some of the parasitic flatworms absorb nutrients directly from the host through their body surface.
However, flatworms like liver fluke have an incomplete digestive system. Specialized excretory cells called flame cells help in osmoregulation and excretion. Sexes are not separate (monoecious); fertilisation is internal and development is through larval stages (miracidium, sporocyst, redia, cercaria). Polyembryony XI Std Biology-Zoology Chapter- is common in some flatworms (Liver flukes).
Some members like Planaria show high regeneration capacity ( Figure . ). Examples: Taenia solium (Tape worm), Fasciola hepatica (Liver fluke), Schistosoma (Blood fluke). .
. Phylum: Aschelminthes (Round Worms) (G. Askes –cavity; helminths – worms) Previously called Nematoda, this phylum is now named as Aschelminthes. The body of these worms is circular (round) in cross section and hence are called round worms.
They are free living or parasitic on aquatic and terrestrial plants and animals. They are bilaterally symmetrical, triploblastic and pseudocoelomate animals with organ system level of organisation. The body is unsegmented and covered by a transparent, tough and protective collagenous layer called cuticle. The alimentary canal is complete with a well developed mouth, muscular pharynx and anus.
Excretory system consists of renette glands. Sexes are separate;