STUDIES ON THE CARDIAC STOMACH OF A STARFISH, PATIRIA MINIATA (BRANDT)

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Patina ininiata, the cushion star or sea-bat of the Pacific Coast of North America, is perhaps best known to zoologists as a ready source of materials for the study of asteroid embryology. Of equal interest, however, are its voracious appetite and its unusual feeding habits. Patina functions as an omnivorous scavenger of both plant and animal materials and to some degree at least as a predator on sessile gastropods. Unlike Asterias and Pisaster, it does not open bivalves; but it has developed to a much greater extent than any of the free-armed starfishes the ability to evert the cardiac stomach through the mouth and to employ it as a feeding organ of great effectiveness. Almost every individual observed undisturbed in a tide pooi is found with its remarkably voluminous cardiac stomach fully everted and widely spread. Specimens maintained in an aquarium spend a good share of the time with their everted stomachs closely applied to the glass, perhaps, as suggested l)y MacGinitie and MacGinitie (1949, p. 227), digesting accumulated growths of diatoms. Bits of kelp or other seaweed, cracked snails, large pieces of mussel flesh, living limpets, or even smaller specimens of Patina are covered and held against the substratum by the body of the starfish, then enveloped by the spreading folds of the everted stomach (Fig. 4) until completely digested. Even small pieces of soft food, which the animal is capable of ingesting, are first covered and wrapped in folds of the stomach before being brought into the cavity of the gut. Such observations suggest that the cardiac stomach of Patina functions in a highly specialized manner, being more frequently and more extensively exposed to the external environnient than the stomachs of other familiar starfishes. It seemed likely that the functional peculiarities of this organ might be reflected in significant structural specializations, probably involving regional differentiation of the stomach wall and a particularly well-developed system of fibrous attachments for holding and retracting the everted stomach. Although Patina is a relatively common intertidal species in much of its range along the Pacific coast, no detailed anatomical or histological studies of its digestive tract appear to have been made. The present 1 These studies form a part of a program of investigation begun at the Hopkins Marine