Introduction.

Stichopus mollis is known from the littoral zone of Central and Southern New Zealand, and from New South Wales to Southern West Australia. It is found on coarse sand and mud in sheltered regions from low-tide level to a depth of 1,530 metres. Previous work on this species, listed-by Mortensen (1925), has dealt mainly with the systematic features of the external appearance and of the calcareous spicules which form the internal skeleton. In his systematic account

of New Zealand holothurians, Dendy (1896) gave a brief account of the gross internal anatomy of S. mollis. He observed that “it. is very difficult to prevent the animal from discharging its viscera when one attempts to preserve it.” No further observations on auto-evisceration in this species, or investigations of the possible regeneration of the organs expelled were made.

Autotomy has frequently been reported in holothurians. The expulsion of internal organs has been demonstrated in most cases as a response to artificial stimuli (Pearse, 1909; Scott, 1914; Kille, 1931, 1935) with fewer cases of its definite occurrence in natural conditions (Minchin, 1892; Bertolini, 1932a; Kille, 1936). Transverse fission of the whole body has been recorded in specimens kept in aquaria (Dalyell, 1851; Chadwick, 1891), but it has also been found to occur in natural conditions (Monticelli, 1896; Crozier, 1917; Deichmann, 1922; Kille, 1936). As far as auto-evisceration is reported, there is considerable variation in the points of rupture, and consequently of the organs expelled by members of different genera of holothurians.

While it has been recognised that regeneration occurs after these cases of autotomy, there are fewer accounts giving precise details of the stages in the reconstitution of organs lost. For Thyone, Scott (1914) and Kille (1935) described stages of regeneration after induced autotomy, while Torelle (1909) described the restoration of parts experimentally removed. Regeneration in Holothuria is described by Bertolini (1932) and Kille (1937). The only accounts of regeneration in Stichopus are those by Bertolini (1930, 1931). describing the condition observed in specimens of S. regalus which were in stages of regeneration when collected from natural conditions, as it was not found possible to keep S. regalus alive for more than a day or so in the laboratory. There has been no account of regeneration in Stichopus at known intervals after autotomy.

The problems of the mechanism of autotomy, and the regeneration of the organs and organ-systems expelled from holothurians have not been fully studied. In some instances, this may be related to experimental difficulties or to insufficiency of material examined. In the case of S. mollis there is ample material available. Evisceration by various stimuli can be induced, and experimentally the animal is a good subject excepting that the rate of regeneration has proven remarkably slow.

In the course of this study it was found necessary to study the normal anatomy and histology of S. mollis. A description of the results is beyond the scope of this paper, but in details essential for a comparison of regenerated with normal tissues, it was found similar to S. chloronotus as described by Sivickis and Domantay (1928). Reference to this paper can be made for a comparison of the regenerated organs of S. mollis with those of normal specimens of Stichopus.