THE APODOTJS HOLOTHURIANS 45 



The gemis Polyplectana has more than 20; the normal number is probably 25, 

 but individuals with as many as 27 are known, and it is common to find speci- 

 mens with less than the normal number. In regard to the arrangement of the 

 tentacles, when 10 are present there are 2 in each interradius; when 12 are 

 present, the right and left dorsal interradii have 3 each; when 13, the addi- 

 tional tentacle is either in the mid-dorsal or in one of the ventral interradii; 

 when 15, there are 3 in each interradius. We have no information in regard to 

 the arrangement of the tentacles in the species with more than 15 tentacles. 

 The shape of the tentacles differs very markedly, but they are usually provided 

 with finger-like outgrowths called digits. Rhabdomolgus is the only genus in 

 which there are no distinct digits whatever. In all other Synaptids there are 

 always at least 2 digits, one on each side, and the number may be as great as 

 30 on each side. In many cases the tentacles are truly pinnate (Plate VI, fig. 

 19), having a tapering shaft with the digits arranged in a simple row along 

 each side. Those near the middle are the oldest and usually the longest, new 

 digits being formed at both ends of the shaft as the animal grows. In some 

 cases the tentacles are very short, and the few digits (2 or 4) arise at the tip 

 end, from what is nearly a common source; such tentacles are called digitate. 

 In the Chiridotinas we have an arrangement of the digits along the margin of 

 a flattened disc which occupies the upper and outer side of the tentacle. The 

 longest digits are at the top, the youngest and smallest at the lower, outer end. 

 Such tentacles are sometimes called peltafo-digitaie. When contracted, the two 

 sides of the disc fold toward each other outwardly, and, tlms folded, the whole 

 disc can be withdrawn more or less completely into the base of the tentacle. 

 The tentacles are sometimes connected with each other at their bases by a 

 more or less evident membranous fold, and in some species the digits are 

 united to each other in a similar manner. So far as known, the tentacles of 

 adult Synaptids are always of equal size. The cases which have been recorded 

 where such was not the case are clearly young individuals, or the observer has 

 been deceived by the unequal contraction of the tentacles. The tentacles are- 

 always hollow, containing, as they do, the tentacular canals of the water-vascu- 

 lar system. The wall of the tentacles is made up of a thin cutis overlying a 

 layer of epithelial cells, beneath which is a connective tissue layer — an arrange- 

 ment essentially the same as in the body-wall. Any calcareous particles pres- 

 ent in the tentacles lie in the connective tissue. The cavity of the tentacle is 

 lined with a thin epithelium, and between this and the connective tissue occur 

 the muscles. Longitudinal muscles are always present, so far as synaptid 

 histologj'' is known, but the presence of circular muscles is a disputed point. 

 They are certainly absent in some species, but seem to be present in Labido- 

 plax digitata and perhaps in some others. The digits of the tentacles have the 

 same structure as the tentacle itself, but the central cavity which each one con- 



