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Process of Auto-evisceration.
After the injection of distilled water S. mollis remains quiescent in a half-extended condition for approximately one minute. A distention of the posterior half of the animal follows, accompanied by contraction of the longitudinal muscle bands and a decrease in volume of the whole animal. The dorsal wall of the cloaca then ruptures immediately posterior to its junction with the base of the respiratory trees. Through this rupture the right respiratory tree is reflected backwards and protrudes through the anus with the tips appearing first. Both respiratory trees appear when the left branch is not connected with the haemal plexus, which is a condition frequently found in smaller, specimens. The alimentary canal protrudes in the from of two loops, the first to appear being the stomach and part of the intestine immediately posterior to it. The second loop contains the intestine with the haemal plexus and usually the left respiratory tree is entangled in this. The first stage in the expulsion of viscera is relatively violent, accompanied by a rapid outflow of coelomic fluid and occupies about 15–20 seconds. A slight relaxation of the body follows and viscera flows slowly, or remains in the same state of extrusion for 30–40. seconds. Another con-
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traction then commences and the remaining portions of the alimentary canal are forcibly expelled with the ruptured anterior end appearing last. The anterior break occurs in the oesophagus about 2 cm. posterior to the water vacular ring. Specimens killed during autotomy showed this region intact, although most of the internal organs were protruding through the cloaca. The rupture in this region apparently occurs late in the process. Separation of viscera from the mesenteries occurs close to the line of junction between the two, which leaves the mesenteries intact. When the branches of gonads are sufficiently long to become entangled among the viscera, they will be carried out with the alimentary canal.
The process of auto-evisceration is the same, irrespective of the type of inducing agent. It is normally completed two minutes after the first appearance of tips of the respiratory trees.
When gonads are insufficiently large to become entangled in the viscera, branches are frequently expelled separately several hours later. In one case a contraction of the body followed by expulsion of branches of the gonads occurred 23 hours after the other organs had been expelled. Gonads less than 2 cm. in length are not expelled. In all specimens examined, it was found that the regions of rupture were constant, so the same organs were always expelled, except for variations in the case of gonads. All the alimentary canal between the regions of rupture, the haemal vessels supplying it, and both respiratory trees are lost.
Fig. 1.—Dissection opened along right dorsal interambulacrum showing structures remaining after auto-evisceration.
A., anus; Cl, cloaca; D.M., dorsomedian (dorsal mesentery; G., gonads; H.R., haemal ring; L.M., left dorso-lateral (lateral) mesentery; L.M.U., longitudinal muscle bands; OES., remnant of oesophagus; R.M.U., retractor muscles of cloaca; R.W.V., radial water vessel; T.A., tentacle ampulla; V.I.V., ventral intestinal haemal vessel; V.M., right ventro-lateral (ventral) mesentery; W.V.R., water vascular ring. X′—X″ distance calculated as percentage of X—X″ in measurements of Fig. 2.
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The body wall, with its attached musculature, the haemal and nerve rings with their radial branches, and the complete water vascular system is left intact. (Fig. 1.) A short length of oesophagus with the corresponding length of the dorsal and ventral intestinal vessels remains, and the cloaca and its retractor muscle fibres are left intact except for the rupture at the anterior end. Short lengths of gonad branches remain on either side of the dorsal mesentery. There is a continuous connection of mesenteries from the lantern to the cloaca. with the free edge delimiting the position of the original alimentary canal.
The dorsal mesentery (Fig. 1) originates along the dorsal interambulacrum from its anterior end and continues posteriorly almost to the cloaca. Along its inner edge the lantern and oesophagus remnant remains supported. The remainder of the oesophagus and the muscular stomach were suspended along it posteriorly. From the posterior end the dorsal mesentery crosses over a longitudinal muscle band and continues anteriorly along the left dorso-lateral interambulacrum. The posterior portion of the stomach and first portion of the intestine were suspended by this section. At the anterior end, the mesentery crosses to the right ventral interambulacrum close to the ventral longitudinal muscle and continues posteriorly to the cloaca. There is a considerable distance between the intestinal attachment and origin of the mesentery from the body wall at the region of its anterior angle. The intestine was therefore suspended considerably further posterior to the lantern than the point of mesentery origin. At this point a distinct junction occurred between small and large intestine distinguishable by colour and texture rather than change in size. The large intestine was suspended from that point to its junction with the cloaca. Regeneration of the alimentary canal occurs along the continuous edge of the mesenteries.