The circulatory system of Pollicipes polymerus exhibits a high degree of organization which precludes it from being referred to as an open system. The system is arbitrarily divided into four parts: (1) the circulation of the peduncle and mantle; (2) the distributive circulation of the body, which provides hemolymph to most of the cephalic gut, to the maxillary gland, and to the cirri; (3) the peripheral circulation which distributes blood from the cirri to the peripheral areas of the thoracic region, to most of the thoracic gut, and from the scutal sinus to the peripheral areas of the cephalic region; and (4) the collecting circulation, which conveys hemolymph mostly from the peripheral circulation of the body to the peduncle. There also may be a circulation that is comparable to the vertebrate lymphatic system. Pumping of hemolymph can be attributed to three pairs of skeletal muscles that compress the dorsolateral channels. These muscles are unique for crustacean muscles in that they do not appear to be striated. The rostral vessel appears to be a vestige of a heart in which the pump muscles have been lost. There is a similarity of the rostral vessel to the heart of Calanus finmarchicus (a copepod). This is additional evidence linking the cirripeds with the copepods within the Maxillopoda. Electron microscope observations of the walls of the midsagittal vessels indicate that there is a more or less random layering of cellular and noncellular elements within the wall. Muscle cells appear to be incorporated in the vessel wall.