In the pericardial cavity of the decapod Crustacea the nerve fibres end in characteristic neuropile-like networks which surround bundles of thicker fibres or spread over the pericardium wall and the heart ligaments. It is proposed to designate the whole system of these elements as 'pericardial organs'.

The arrangement of the elements constituting the pericardial organs varies in different groups of the Decapoda. In the Brachyura the bundles of fibres surrounded by neuropiles form conspicuous trunks linked with each other; the main parts of these trunks are situated at the openings of the branchiocardiac veins into the pericardial cavity. In Eupagurus bernhardus two main trunks flank the heart giving off branches which spread in various directions and form at some places plexuses on the pericardium wall. In Leander serratus the pericardial organs are represented by several trunks running near the lateral wall of the pericardium in the dorsal direction and uniting by archshaped branches. In Homarus vulgaris there are four comparatively much shorter trunks situated at the lateral wall of the pericardial cavity, but plexuses of neuropile-like nerve fibres spread over large areas.

The trunks of the pericardial organs are suspended in the pericardial cavity in such a position that they may be bathed on all sides by the blood passing from the gills. The plexuses extending over the membranes are situated in places also exposed to the blood stream. There is evidence indicating that the pericardial organs produce substances influencing the heart-rhythm. It is assumed that these substances are liberated into the blood by a process of neurosecretion occurring at the terminations of the nerves.

When a striated muscle is stimulated at one point, the excitation travels away from that point at a finite speed. The whole of the muscle is active when the wave of contraction has travelled the length of the muscle. Doubt has been cast as to whether the electrical wave associated with membrane excitation travels at the same velocity as the wave of contraction.

A new longifurcous, pharyngeate, distome cercaria has been found to develop in the marine bivalves Hiatella arctica (L.) and H. striata Fleuriau. This larva was present in 12 of 200 specimens of H. arctica collected from Drake's Island during the summer months (May-August) of 1952,.while the same parasite was found in 15 of 100 specimens of the limestone borer, H. striata, dredged from Plymouth Sound during June and July of the same year.