An invagination of the intestinal epithelium that emanates from a lamina of mesenchymal cells located between the dorsal aorta and the upper intestine and constitute one of the two principal hematopoietic organs at the ammocoete stage of development. [ http://en.wikipedia.org/wiki/Typhlosole https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2093943 ]