HOLOTHURIOIDEA 351 



skin in Canada balsam or dissolving pieces of skin by means of 

 potash or hypochlorite of sodium (" Eau de Javelle "), and thus 

 isolating them. 



Under the epidermis comes a layer of circular muscles, in- 

 terrupted by the five strong longitudinal or radial muscles, which 

 run through the whole length of the bod}", from mouth to anus ; 

 each radial muscle may be divided into two parallel halves. 

 Retractor muscles may occur ; they issue from the radial muscles, 

 at various distances from the anterior end, passing through the 

 body cavity to the calcareous ring. They serve to retract the 

 anterior end of the body, with the mouth and the tentacles. The 

 retractile part of the body wall, the " introvert ", is usually 

 thinner than the rest of the body wall and the calcareous bodies 

 contained in it are usually somewhat scarcer and often somewhat 

 different in shape from those of the rest of the skin. The tuhe- 

 feet are principally arranged in five double series, but they often 

 become pluriseriate, especially in the ventral (trivial) radii, and 

 they may even spread over the whole body. They may be 

 confined to the ventral side alone, or they may be present in the 

 shape of papillae, without sucking disk, on the dorsal side. They 

 always issue directly from the surface, are never placed in 

 furrows. Tube-feet are entirely lacking in the Synaptids, and in 

 the Molpadids they are represented only by papillae around the 

 anus. The tentacles may be bush-shaped, shield-shaped, feather- 

 shaped or simply finger-shaped. Their number is mostly 10, 12, 

 15, or 20, but it may sometimes be 8, 11, 25, or even 30. Their 

 shape is very constant in the various groups and affords a 

 systematic character of primary importance. 



The mouth usually is at the anterior end, but may in some 

 forms be situated more or less ventrally, in others dorsally. It is 

 unarmed. The oesophagus, on the other hand, is surrounded by a 

 structure highly characteristic of the Holothurians, the calcareous 

 ring (Fig. 211). This consists of usually 5 radial and 5 interradial 

 pieces, of rather different form in the various groups and therefore 

 of considerable importance for classification. The radials mostly 

 differ in shape from the interradials, and usually have a notch 

 in the anterior edge over which the radial muscle and radial 

 nerve pass. The notch may close itself and thus form a hole 

 through which the radial nerve passes (some Synaptids). The 

 radials may have a posterior, bipartite prolongation, sometimes 

 {Pseudocucumis) also the interradials may have a posterior pro- 

 longation. These prolongations may be articulated, as also the 

 whole calcareous ring may be divided into a number of smaller 



