Lesson 2 of 12 · 6 min
Symmetry and germ layers
NCERT §4.1.2, §4.1.3
Ammu cuts a paper outline of each tide-pool animal and tries to fold it into matching halves. The sponge will not fold into equal halves any way she tries. The jellyfish folds evenly along every line she draws. The crab folds evenly along only one.
The lesson in notes
In short
Sponges are mostly asymmetrical: no plane through the centre cuts them into two equal halves.
Radial symmetry: any plane passing through the central axis gives two identical halves. Coelenterates, ctenophores and adult echinoderms are built this way.
Bilateral symmetry: only one plane divides the body into identical left and right halves, as in annelids and arthropods.
Diploblastic animals develop from two embryonic layers, an outer ectoderm and an inner endoderm, with an undifferentiated mesoglea between them. Coelenterates are the example.
Triploblastic animals have a third germ layer, the mesoderm, between ectoderm and endoderm. Every phylum from Platyhelminthes to Chordata is triploblastic.
Ctenophores, like cnidarians, are diploblastic; the diploblastic phyla are the two with radial symmetry and tissue-level organisation.