Includes the helminthes of lowest organisation. The animal of this phylum are also known as flatworms because they are dorso-ventrally flattened.
Definition-The flatworms are dorso¬ventrally flat, bilaterally symmetrical, triploblastic with mesenchyma filling the spaces between the various organs. They are devoid of definite, anus, circulatory system, skeletal system and respiratory system, Excretion by flame cells.
Phylum Platyhelminthes General Characters-
1. Symmetry: Bilateral with dorso¬ventrally flattened body with head present anteriorly.
2. Shape: Worm like but may be ribbon like or broad leaf like.
3. Size: Generally of moderate size but few are of 10•15 meters.
4. Adhesive organs: Secretion or I and suckers; hooks etc.
5. Body covering: A syncytial, cellular and partly ciliated epidermis with cuticle only.
6. Skeleton: Absent but contains organs which are cuticularised with scleroproteins only. These hard organs are provided with hooks, spines, thorns, spicules etc. which are of cuticle.
7. Digestive system: Absent in acoela and flatworms but rest of these animals are provided with digestive system.
8. Excretory system: By protonephridia which are provided with special type of cells - the flame cells.
9. Nervous system: Primitive type. Mainly consists of cerebral ganglia or brain.
10. Sex: Hermaphrodite (mostly).
11. Reproductive system: Well developed. Asexual reproduction in planaria is by fission and sexual reproduction by generalized formation of ova and sperm fusion.
12. Development: With few larval stages.
Class 1-Turbellaria e.g. convoluta, segmentata etc.
Class 2 -Trematoda e.g. polystoma, schistosoma, fasciola etc.
Class 3-Cestoda e.g. phyllobo¬thrium, taenia, protocephalus etc.
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