Swimming larvae of Phallusia mamillata, Ascidia malaca and Ascidiella aspersa, show, as a result of certain treatments (Nadi and benzidin-reaction; toluidin-blue colouration) some peculiar cells between the ventral surface of the intestine and the epidermis. These cells, which have been described by Ries in the past, are not, as supposed, mesenchymal cells in migration from the tail to the base of the palps, but entodermal cells. They contain peroxidases, which probably are implicated in the elaboration of a mucopolysaccharide substance which is utilized for the construction of the tunic of the metamorphosing ascidia.