Studies on the cardiac stomach of the starfish, Asterias forbesi
Anderson, John Maxwell
Biol Bull 107(2): 157-173
The outer surface of the cardiac stomach bears a complex, specifically distributed system of branching fibers of muscular and connective tissue, inserting ultimately on the muscular and connective-tissue coats of the stomach wall. These fibers originate from 5 band-like nodules, points of attachment to the stomach of the paired retractor strands in the arms. These strands, the extrinsic parts of the retractor system, form the upper borders of triangular, mesentery-like sheets binding them to the ambulacral ossicles. The extrinsic parts consist chiefly of collagenous fibers and contain very scanty musculature; it is suggested that they serve merely to anchor the 5 nodular regions of the stomach, against which the intrinsic retractor fibers of the stomach wall may stretch and pull at eversion. It is also suggested that the more significant aspects of retraction involve not the very slightly contractile extrinsic fibers, but the widely distributed, muscular branches of the intrinsic mechanism. Modern authors have disregarded the intrinsic parts of the system and commonly refer to the extrinsic strands as retractor muscles. A summary of the development of this narrow emphasis is presented. Coincident with the terminal branches of the intrinsic fibers occurs a series of branching channel-systems on the inner surface of the stomach. These gutters open at the junction between stomach and esophagus and run upward, coalescing to form larger channels higher in the stomach. The bottoms of the gutters are lined by typical, non-specialized, tall epithelial cells with small, ovoid nuclei and bearing each a single flagellum. Ridges between the grooves are covered by specialized cells, with long, spindle-shaped nuclei and 2-6 flagella each. Smith has suggested that these cells represent sensory receptor elements. They are localized in a part of the stomach not strongly everted at feeding, but forming the lining of the narrow gullet through which all ingested food must pass. Such cells are not found in the cardiac stomach above the lower parts of the channel systems, but similar cells occur in the lining of the "esophagus" which appears to form the major share of the vesicles of the everted stomach. In the stomach generally, currents sweep in an upward direction, from mouth to anus; in the channels, flagella whisk particles not upward, but outward into the lumen. Certain secretory cells localized on the walls of the gutters may contribute their products to these centripetal currents; their function is unknown. Acetylcholine and epine-phrine act antagonistically in controlling the opening and closing of the mouth. Adrenal cortical hormone (ACH) opens the mouth by causing contraction of the radial musculature of the peristome and esophagus; epinephrine reverses this action by bringing the circular muscles into contraction. It is suggested that these effects may reflect reciprocal innervation of these muscular layers from different components of the circumoral nerve ring. No action of chemical mediators was demonstrated on elements of the retractor system, intrinsic or extrinsic, although coordinated action is a necessity.