Free living or

Commensal or

parasitic forms

Tissue-organ grade of

organization

Triploblastic:

ectoderm, mesoderm

& endoderm

Bilaterally symmetrical

Dorsoventrally

flattened

Well defined ventral

surface (mouth &

gonopore)

Body unsegmented

except in class

Cestoda

Acoelomate:

without any body

cavity

Body spaces:

mesenchyma

/parenchyma

Adhesive

structures: hooks,

spines & suckers

Epidermis:

Cellular/syncytial,

frequently ciliated

Absent in Some

Muscular system:

mesodermal

Origin

Longitudinal,

circular & oblique

muscle layers

Digestive system:

Branched &

incomplete without

anus

Altogether absent

in Acoela & Cestoda

Skeletal, respiratory

circulatory systems

wanting

Excretory system:

Lateral canals Flame

cells

Absent in some

primitive forms

Nervous system:

primitive, ladder-like

A pair of anterior

ganglia with

longitudinal

nerve cord

(transverse nerves)

Sense organs:

Simple eye spots or

photo receptors

(free living)

Mostly monoecious

(Hermaphrodite)

Gonads,

gonoducts

accessory

organs

Eggs mostly

devoid of yolk

Yolk: by yolk or

vitelline glands

Fertilization:

Internal or cross or

self

Development

may be direct or

indirect

Indirect in

endoparasites with

a complicated life

cycle

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