The embryonic layer found only in bilaterally symmetrical eumetazoans is the_

The embryonic layer found only in bilaterally symmetrical eumetazoans is the mesoderm.

Here are the steps to answer this question:

1. Understand the terms:

   • Embryonic layer refers to the layers of cells in an embryo that will develop into different types of tissues and organs.
   • Bilaterally symmetrical eumetazoans are animals that are symmetrical along a central axis and have a true body cavity. This includes most animals, excluding sponges and cnidarians.

2. Identify the layers:

   • There are three embryonic layers: ectoderm, mesoderm, and endoderm. The ectoderm forms the skin and nervous system, the endoderm forms the digestive system, and the mesoderm forms the muscles, skeleton, circulatory system, and other organs.

3. Identify the unique layer:

   • Of these three layers, the mesoderm is unique to bilaterally symmetrical eumetazoans. Sponges and cnidarians, which are not bilaterally symmetrical, do not have this layer.

So, the embryonic layer found only in bilaterally symmetrical eumetazoans is the mesoderm.