Some new data on skin-digestion and absorption in urchins and sea stars (Asterias and Henricia)

An adult Psammechinus miliaris, and another specimen 8 mm in diameter were both fed apically by deposition of a protein-powder extracted from a heavily ¹⁴C-tagged urchin. The organs of the former were counted and the latter was totally sectioned and autoradiographed. The gut was more labelled than expected after an apical meal. This could be explained by a pumping of some nutritive powder into the gut, the bright marking of the superficial tissues being obviously due to direct uptake. The localized proteolysis produced by a podial disc of P. miliaris on a black gelatin-film may be due to the secretion of special cell-masses just underlying the calcareous plates of the disc. Extracts from mucus, small spines and dorsal podia from the anterior furrow of Echinocardium cordatum produce a conspicuous digestion of Remazol-brilliant blue hidepowder in vitro. Several in vivo tests with artificial substrates suggest the existence of a trypsin-like enzyme and not chymiotrypsin in the mucus. Fresh frozen sections in podia of Asterias rubens treated with A.S. naphthol-propionate showed that special cells, scattered in the connective tissue, gave a dark-red stain. Strains of red granules issued by them found their way to the surface of the disc between the columnar epithelial cells. These chimiotrypsin-positive cells and their products seem identical to those seen by Chaet and Philpot (1964) in A. forbesi. These authors did not suspect their digestive function. The nutrition of Henricia sanguinolenta through the body wall was tested either with radioactive diatoms or ¹⁴C-labelled amino acids in sea water selectively offered to one or two arms. Both countings and autoradiographs demonstrated the importance of skin-digestion and absorption, especially in the ambulacra. The absorbed nutrients reach the muscle layer, not only the epiderm as mentioned in Ferguson's (1967) esperiments.