Autoevisceration and Regeneration in Stichopus mollis (Hutton)

Abstract

Autoevisceration has been recorded in a number of genera of holothurians, and consists of the ejection of the alimentary canal and other organs in response to certain stimuli. The stages in this process have not been recorded very fully, the main study (Minchin, 1892) having been made on the ejection of Cuvierian organs. As autoevisceration occurs in Stichopus mollis (Hutton), this species has been used in a study of this process and of the stages in the regeneration of lost viscera. S. mollis is restricted to the littoral zone of New Zealand and Australia, whence it is found on coarse sand in sheltered regions. Previous work on this species has dealt mainly with descriptions of its external appearance, and of the calcareous spicules which form the internal skeleton. In the present study, a description of the normal anatomy is first given in order to compare the tissues of normal animals with that described later from regenerating Specimens. Organs lost by S. mollis during autoevisceration are the alimentary canal, both respiratory trees, the intestinal vessels, and in some cases the gonads. A study of the tissue regenerated after autoevisceration has been carried out. Previous descriptions of regeneration in holothurians have been mainly confined to gross anatomical changes and differ in details according to the species studied. In S. mollis is has been found that the regenerated alimentary canal and associated organs are derived from the mesentery remnants, which although wholly mesodermal in origin, can yet give rise not only to mesodermal tissue, but also to tissue that takes the place and fulfils the function of tissue which is normally endodermal in origin. The regenerated alimentary canal is proliferated from along the whole length of the continuous mesentery edge, thus retaining the same form of loop as is found in normal specimens. Histological comparisons of the regenerating tissue with the tissues of the normal animal have been made. From this comparison it has been possible to trace the stages in the regeneration of viscera lost by autoevisceration.