Flatworms are in the phylum Platyhelminthes. Most flatworms are dorsal ventrally flattened and have bilateral symmetry. Bilateral symmetry means an animal has a right and left side and is important in the evolution of animals because it allows for different parts of the body to have specialized structures, like a head with specialized sense organs, limbs, and other organ systems located in certain parts of the body.
Flatworms have three germ layers. Like cnidarians flatworms have a layer of ectoderm and endoderm, but also a third germ layer called mesoderm. In lab, we will be examining a live free living flatworm called Planaria, make sure you look for cephalization, which means they have a head with specialized sense organs (eyespots, auricles) to move head first through the environment. Further, flatworms have several organ systems, including a well developed excretory system, nervous system and reproductive system. Flatworms lack a respiratory and ciruclator system. Flatworms are acoelomate because they lack a body cavity (fluid filled layer) and their body construction is solid.
Germ layer organ development:
Planaria eating egg yolk from lab
In lab, we fed planaria egg yolk. Check out the pictures of the gastrovascular cavity on the right. One picture will be of a live planaria that has been starved (gastrovascular cavity is empty) and then the other pictures will have the yellow egg yolk granules in the gastrovascular cavity