The endoderm allows them to develop true tissue. This includes tissue associated with the gut and associated glands. The ectoderm on the other hand gives rise to the epidermis, the nervous tissue, and if present, nephridia.
Simpler animals, such as sea sponges, have one germ layer and lack true tissue organisation.
There is no pharmaceutical or device industry support for this site and we need your viewer supported Donations | Editorial Board | Governance | Licensing | Disclaimers | Avoid Plagiarism | Policies