The anatomy of the hadal, cocculinid limpet Macleaniella moskalevi is described. This species has a small, symmetrical, cap-shaped shell with a prominent internal transverse septum. As in other cocculinid genera, broad oral lappets and epipodial tentacles are present. Vestigial eyes are also present, but lack pigment and are modified into the basitentacular gland. The unique aortic vessel and associated hemal gland are present. Mantle cavity morphology is characteristic of cocculinid taxa; the cavity contains a pseudoplicatid gill, hypobranchial gland, and shallow brood pouch. Adults are simultaneous hermaphrodites, possessing a gonad with distinct egg and sperm producing regions; the common gonoduct is glandular. The single receptaculum seminis is connected to the anterior gonoduct via an enclosed duct. An enlarged right cephalic tentacle is inferred to function as a copulatory organ. The digestive gland occupies that part of the visceral mass confined above the septum and opens by way of two ducts into the distal esophagus. The anterior digestive tract is characterized by the presence of extremely well-developed esophageal pouches, a cuticularized sublingual pouch, and an unpaired jaw. A subradular sense organ and salivary glands are lacking. M. moskalevi is remarkably unique among previously described cocculinids in the anatomy of the reproductive tract and of the organs associated with the mantle cavity. In particular, the conformation of the nervous system in this species is unknown among cocculinids described thus far. Additional key words: Mollusca, deep-sea Cocculinids are limpet-like gastropods that are found in deep-sea habitats worldwide and are known primarily from bathyal depths. Macleaniella moskalevi LEAL & HARASEWYCH 1999 and Fedikovella caymanensis (Moskalev 1976) remain the only cocculinid species to date that are known to inhabit the hadal zone. Apart from the genus Teuthirostria, which inhabits chitinous cephalopod beaks, cocculinids are exclusively associated with submerged wooden substrates. Despite this consistent and predictable substrate association, it remains unclear whether cocculinids obtain nourishment directly from their substrate or from microbes, detritus, or other food resources associated with the surfaces on which they are found. Ever since their discovery and first description (Dall 1882), cocculinid species have intrigued their investigators with unique combinations of features. These features have proven problematic for the taxonomy of cocculinid limpets because they conflict with the suites a Author for correspondence. E-mail: eestrong@ gwis2.circ.gwu.edu of characters thought to be diagnostic at higher taxonomic levels. Difficulty in determining the taxonomic placement of cocculinids has been exacerbated by the common practice of placing any deep-sea limpet with a modified archaeogastropod organization in the same taxonomic group. Thus, the phylogenetic affinities of the family have remained ambiguous, a fact reflected in their fluctuating taxonomic status. Based on his anatomical investigations of Cocculina, Thiele (1903), stressed the isolated position of this family, but inferred a close relationship to Neritopsina. Later, Thiele (1908) suggested a close relationship between Bathysciadium and Cocculinidae and erected the superfamily Cocculinoidea to include the families Cocculinidae (Cocculina), Lepetellidae (Bathsciadium, Lepetella), and Addisoniidae (Addisonia). This scheme was expanded with the description of material collected during the latter half of the nineteenth century, but remained largely unchanged for the next 80 years. By the 1980's the Cocculinoidea had grown to include nine families representing a variety of deep-sea limpet-like forms: Addisoniidae (Dall 1882), Bathypeltidae (Moskalev 1971), Bathyphytophilidae (MosThis content downloaded from 207.46.13.129 on Sat, 25 Jun 2016 06:16:26 UTC All use subject to http://about.jstor.org/terms