In the first part of this paper the anatomy and physiology of the Ommatopleans is described from the typical form O. alba, the variations and peculiarities presented by other genera and species being contrasted therewith. The descriptions are grouped under the following heads:—Dermal Tissues; Proboscidian Sheath; Proboscis; Digestive, Circulatory, and Nervous Systems; Cephalic Furrows, Pits, and Glands; Organs of Reproduction and Development.