STRUCTURE AND FUNCTION IN THE PYLORIC CAECA OF ASTERIAS FORBESI

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Abstract
Histological and histochemical investigations of the pyloric caeca in Asterias forbesi reveal that their walls are composed of an outer splanchnic peritoneum made up of small, flagellated, cuboidal cells; layers of muscular, nervous, and connective tissue fibers variously developed; and an inner epithelium generally composed of very tall, slender, flagellated cells bearing a dense brush border. Certain special areas (Tiedemann's diverticula) are lined with closely-packed cells termed "special current-producers," functioning to maintain directed movements of the contents of these central tubular cavities. Interspersed among these cells are numerous mucus gland cells. The walls of the lateral outpocketings of the central cavity are lined by an epithelium consisting of (a) secretory cells, producing conspicuous secretory (zymogen) granules and another product contained in small, clear vacuoles; and (b) storage cells, containing abundant lipids (almost entirely neutral fat), glycogen, and a polysaccharide-protein complex resistant to diastatic digestion. Mucus gland cells are less numerous in these areas. "Special current-producers" do not contain appreciable amounts of reserve substances, and the regions of the epithelium of which they form the lining do not show the vigorous alkaline phosphatase activity characteristic of the free border in the storage-cell areas. Starvation for 6 to 8 weeks results in a complete disappearance of all reserves but, does not impair the secretory activities of mucus gland cells or of zymogen cells. Previous works on the structure and functions of the Asterid pyloric caeca are summarized, and from all sources of evidence it is concluded that these organs function in digestion of food which passes into them from the stomach, in absorption of the products of digestion, and in storage of reserves. Transfer of nutrients to the coelomic fluid, for distribution throughout the body, is also a feature of their activities.

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