A new body plan The pseudocoelomates

The pseudocoelomates Tube with in a tube plan Gut has separate entrance and exit Body space is lined by mesoderm, but only on the outer side, the endoderm lacks a mesodermal covering Constant cell number, growth of cell size rather than cell number

The pseudocoelmate phyla Nematoda Caenorhabditis elegans Rotifera Acanthocephala

The Nematoda Bilaterally symmetrical round worms Lack a cellular epidermis, but instead have a cuticle. Need to molt cuticle for growth to occur Use a hydrostatic skeleton, but in this case the fluid skeleton is separate and distinct from the gastrovascular cavity Only have longitudinal muscles