Metamerism and Germ Layers

Metamerism

When the segmentation in bilateral animal such as annelids involves a longitudinal division of the body into a linear series of similar sections or parts is termed Metamerism or metameric segmentation. e.g. Annelids,Arthropods, Chordates and Cestodes.

Fig. A typical metameric animal such as an annelid worm

1. Annelids show both external and internal segmentation.

2. Arthropods show only external segmentation.     

3. The segmentation in flatworms such as tapeworm is termed as pseudometamerism.

Germ Layers

During early stages of embryonic development the cells are arranged in groups forming germ layers. They appear in gastrula stage.

1. Diploblastic animals – Having two distinct germ layers i.e. outermost ectoderm and innermost endoderm e.g. Cnidarians and porifera.

2. Triploblastic animals – Having three distinct germ layers i.e. ectoderm, mesoderm and endoderm. The mesodermal layer always lies between    ectoderm and endoderm e.g. Most metazoans (Bilateria).