Here are the key characteristics and examples of protosomes:
1. Spiral Cleavage: During embryonic development, protostomes exhibit spiral cleavage, a pattern where the cells divide asymmetrically, resulting in a spiral arrangement of blastomeres.
2. Early Formation of the Mouth: In protostomes, the mouth is formed first during embryogenesis, before the anus. This contrasts with deuterostomes, where the anus develops first.
3. Trochophore Larva: Many protostomes, especially marine species, have a characteristic larval stage called the trochophore larva. The trochophore is free-swimming and has a ring of cilia used for locomotion and feeding.
Examples of protostomes include:
- Platyhelminthes (Flatworms): Flatworms are acoelomate animals with a simple body plan. They include parasitic species like tapeworms, as well as free-living forms like planarians.
- Nematodes (Roundworms): Nematodes are pseudocoelomate animals with a cylindrical body. They include parasitic species like hookworms, as well as free-living forms found in soil and aquatic environments.
- Annelids (Segmented Worms): Annelids are coelomate animals with segmented bodies. They include earthworms, leeches, and tube worms.
- Mollusks: Mollusks are coelomate animals with soft bodies and a variety of forms, including gastropods (snails and slugs), bivalves (clams and mussels), and cephalopods (squids, octopuses, and cuttlefish).
- Arthropods: Arthropods are the largest and most diverse group of protosomes. They are characterized by segmented bodies, jointed appendages, and an exoskeleton. Arthropods include insects, crustaceans (crabs, lobsters, shrimp), arachnids (spiders, scorpions, ticks), and myriapods (centipedes and millipedes).
Protostomes represent a wide range of animal diversity, from simple parasitic forms to complex and highly adapted organisms like insects and cephalopods. Despite their differences, they share the defining features of spiral cleavage, early mouth formation, and the presence of a trochophore larva in many marine species.