This paper forms the first half of Part I. of a research into the minute anatomy, physiology, and development of Patella vulgata. The paper aims at giving a complete account of this form in both its physiological and morphological aspects. It treats, first of all, with the external features and rough anatomy. The epidermal system is then treated in detail. Some interesting modifications of the superficial epithelium are described. The epithelial cells are usually elongated columnar, with processes connecting them with a dense subepithelial layer of connective tissue. The attachment of the circular muscle is covered with modified epithelium, the epiblast cells not being absent in that region, as is said to occur in the case of the adductor muscles of Anodon. The modified epithelium at one spot near the base of the tentacle functions as an eye. The cells are there pigmented, and the cuticle is modified so as to form a double layer, with intervening trabeculae. The retinal epithelium springs from a nerve plexus, which is supported by a mesh-work of connective tissue fibres, with large lacunas.