The Developmental Biology course, and this site, draws upon a variety of invertebrate model organisms, including bacteria, amoeba, and fungi. A phylogenetic tree classifying eukaryotes into six supergroups is shown here, after Simpson and Roger 2004. Although some of these supergroups (esp. Archaeplastida and Chromalveolata) are controversial, most of the model organisms on these pages are in the Opisthokonta supergroup, as metazoans. Dictyostelium (in the Amoebozoa), Bacillus subtilis and flowering plants are the exceptions.

Eukaryota tree, from Wikimedia Commons
- Fertilization: avoiding polyspermy
- Primary and secondary mesenchyme cells
- The Role of Vasa protein accumulation
Some beautiful images of marine invertebrate embryos:
http://jellybiologist.com/2013/06/24/beautiful-embryo-photos/