Introduction :
The Metazoa or the multicellular animals have achieved their structural diversity by varying their cells that have become specialized to perform different functions. These cells are normally incapable of independent existence. Characterized metazoans. Members of Metazoa possess a complex multicellular structural organization which may include the presence of tissues, organs and organ systems. In the life history of metazoans, typically a fertilized egg passes through a blastula stage in the course of its early embryonic development before changing into an . adult.
Since metazoans are multicellular they are relatively larger in size than unicellular protozoans. Naturally, their nutritional requirements are more and they have to search for food. Consequently, locomotion in metazoans is highly developed and for this purpose they have evolved contractile muscular elements and nervous structures. .
GENERAL CHARACTERS OF METAZOA
1.Metazoans are multicellular animals. 2. Metazoans are generally seen in naked eyes.
3. Body of Metazoa is differentiated into cells which may transform into tissues, organs and systems in most cases.
4. Single animal can perform different types of functions by different systems in most groups.
5. Metazoan cells are interdependent and cannot survive in isolated condition.
6. Individual cell of Metazoa is covered by also cell membrane or plasmalemma.
.7 Pellicle is absent in Metazoa.
8. Cytoplasm is present in Metazoa.
9. Chloroplast is present in some species (sponges).
10. Contractile vacuoles found only in freshwater sponges.
11. Many cells are mono or multi-ciliated.
12. Cilia and flagella have same ultra-structures.
13. Digestion intracellular or extracellular or both in some.
14. Food vacuole is absent in Metazoa.
15. Lower groups of metazoans do not possess circulatory, respiratory and excretory structures
16. Haemoglobin, haemocyanin, haemoerythrin, and chlorocruorin—all respiratory pigments present in many groups of Metazoa.
17. Gonads present except a few lower metazoan groups.
18. Motile larvae in their life cycle.
19. The developmental stages possess the embryonic blastula and gastrula stages.
20. Colonial organization is prevalent in some sponges, cnidarians, lophophorates and in some lower chordates.
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