[Summary, printed on p. 345]. (1) The Pacific species, P. hedleyi, is to be associated with the two European species P. minuta and P. sarniensis in the subgenus Ptychodera (sensu stricto), especially characterised by the rudimentary character of the genital pleura.
(2) In the possession of a continuous axial canal in the dorsal nerve cord, opening both anteriorly and posteriorly, P. hedleyi agrees with P. sarniensis, while in the possession of two longitudinal epidermal stripes overlying the two ciliated grooves of the intestine, it agrees with P. australiensis.
(3) P. hedleyi exhibits affinities with the members of the subgenus Chlamydothorax (P. erythrcea, P. bahamensis, and P. flava) in the possession of a dorsal thick-walled glandular division of the intestine just behind the branchial region. It is suggested that this dorsal diverticulum may, in P. hedleyi, have some digestive function.
(4) As regards the degree of development of the dorso-ventral muscle plate, P. hedleyi may be best compared with Balanoglossus kupfferi.
(5) P. hedleyi differs from all hitherto described species of the genus Ptychodera, and agrees with the members of the genus Balanoglossus in the absence of a circular musculature in the trunk.