Protostome 32 Animals 32 Protostome Animals • 32.1 What Is a Protostome? • 32.2 What Features Distinguish the Major Groups of Lophotrochozoans? • 32.3 What Features Distinguish the Major Groups of Ecdysozoans? • 32.4 Why Are Arthropods So Diverse? 32.1 What Is a Protostome? Two triploblastic clades of Bilateria: protostomes and deuterostomes diverged in Cambrian, over 505 mya  Protostomes have far more species and individuals  In most, embryonic blastopore becomes mouth In insects, no blastopore forms  www.emc.maricopa.edu/faculty/farabee/BIOBK Figure 31.1 The Phylogeny of Animals 32.1 What Is a Protostome? Two major clades of protostomes mostly based on DNA sequences:  Lophotrochozoans  Ecdysozoans Figure 32.1 Phylogenetic Tree of Protostomes (Part 1) Figure 32.1 Phylogenetic Tree of Protostomes (Part 2) 32.1 What Is a Protostome? Other general characters of protostomes  bilaterally symmetrical  anterior brain that surrounds the entrance to digestive tract  ventral nervous system with paired or fused longitudinal nerve cords http://organicwaste.com.au/wp- content/uploads/2009/06/earthworm-anatomy.gif 32.1 What Is a Protostome? Common ancestor had a coelom, but some lineages have modifications:  Flatworms are acoelomate (coelom was lost)  Some lineages have pseudocoelom  Arthropods — the body cavity became a hemocoel or blood chamber, part of an open circulatory system http://science.kennesaw.edu/~jdirnber /InvertZoo/Tree/coelom.jpg 32.1 What Is a Protostome?  Mollusks have an open circulatory system similar to the hemocoel but retain vestiges of an enclosed coelom http://kentsimmons.uwinnipeg.ca/16cm05/ 1116/33-16-MolluskBodyPlan-L.gif