The functional morphology of Parilimya fragilis (Bivalvia: Parilimyidae nov. fam.) with a discussion on the origin and evolution of the carnivorous septibranchs and a reclassification of the Anomalodesmata
Authors: Morton Brian 
Published: August 2010

The anatomy of the type specimen of Parilimya (formerly Panacca) fragilis (Grieg, 1920) is described. This anomalodesmatan bivalve lives at great depths in the N. Atlantic as do other members of this and other genera. These genera are generally isolated and are described and discussed for the first time. Parilimya gives its name to a new family (herein erected)—the Parilimyidae—which also possesses a number of other genera i.e. Panacca, Notomya and Nipponopanacca though it is possible that these genera are synonymous with Parilimya. The Parilimyidae are considered to constitute the second family, the other being the Pholadomyidae Gray 1847 represented by Pholadomya candida Sowerby 1823, only of the superfamily Pholadomyacea Gray 1847.

P. fragilis has a very thin rounded and ridged shell with a small external ligament overlain by periostracum. There is a small foot but no pedal retractor muscles and thus probably lies passively buried in soft substrates. It possesses a long, wide‐mouthed inhalant siphon that is used to catch large food particles, probably either dead or living invertebrates. Siphonal expansion is by the pumping of blood into siphonal sinuses and retraction by a pair of taenioid muscles which are elongate siphonal retractors having separate insertions on the shell valves. Pholadomya possesses very small taenioid muscles. Though the plicate ctenidia are of the normal anomalodesmatan type, i.e. with an outer demibranch comprising the descending lamella only and the inner demibranch entire, the palps are small and muscular with few ridges. They probably function by pushing food into the wide, ridged, oesophagus. The stomach is of Type II (Purchon, 1956b), with only two large apertures into the digestive diverticula, a relatively large gastric shield, a small crystalline style arising from a ventral conjoined style sac and mid gut and a posterior deeply ridged extension. The stomach is heavily muscularized as is the whole of the gut. Such a stomach is typical of the septibranch anomalodesmatans and is strongly indicative of a diet of large animals.

Parilimya produces encapsulated large eggs, which are brooded within the ctenidia. Spermatozoa also occur within the ctenidia and are possibly nourished by ctenidial glands until fertilization occurs. Simultaneous hermaphroditism and, probably, a short larval life are adaptations to ensure survival in deep waters.

The anatomical features of Parilimya adequately preview conditions in the families comprising the septibranchs i.e. the families Verticordiidae, Cuspidariidae and Poromyidae. The posterior septal muscles of the Cuspidariidae can be derived from the taenioid muscles of Parilimya. The verticordiid Lyonsiella fragilis also possesses taenioid muscles of characteristic form. The external ligament of the Poromyidae, the septibranch family with the oldest fossil history, is also possessed by Parilimya and is sufficient to separate this family from others within the Poromyacea. The Verticordiidae and Cuspidariidae both possess a sunken ligament with a lithodesma. Although members of this group may thus have a degree of affinity, I prefer to separate them at the superfamily level.