Question

1. why are earthworms and insects called \protostomes\?

Explanation:

To determine why earthworms (annelids) and insects (arthropods) are called protostomes, we analyze their embryonic development:

1. Cleavage Type: They undergo determinate (mosaic) cleavage, where early cell fates are fixed (e.g., a 4 - cell embryo’s cells cannot develop into a whole organism if separated).
2. Blastopore Fate: The blastopore (first opening of the gastrula) develops into the mouth (in deuterostomes, it becomes the anus).
3. Coelom Formation: They use schizocoely to form the coelom (a fluid - filled body cavity): solid blocks of mesoderm split to create the coelom, unlike deuterostomes (enterocoely, where the coelom forms from outpocketings of the archenteron).

These shared embryonic traits (determinate cleavage, blastopore → mouth, schizocoely) define the protostome clade, to which both earthworms (Annelida) and insects (Arthropoda) belong.

Answer:

Earthworms (Annelida) and insects (Arthropoda) are called protostomes because of their shared embryonic development traits:

• They undergo determinate (mosaic) cleavage (early cell fates are fixed).
• The blastopore (first embryonic opening) develops into the mouth (in deuterostomes, it becomes the anus).
• The coelom forms via schizocoely (mesoderm splits to create the coelom, unlike deuterostomes’ enterocoely).

These traits define the protostome evolutionary lineage, uniting earthworms and insects in this group.