Dictionary.net

• The anterior perforated depression communicating with the enteric cavity of many Coelenterates, ectoderm-lined since formed by invagination; the anterior pitted-in portion of the embryonic gut.

1920 - A dictionary of scientific terms.
By Henderson, I. F.; Henderson, W. D.

• An invagination of the embryonic ectoderm whence the mouth-cavity is formed.

1898 - American pocket medical dictionary
By Willam Alexander Newman Dorland