THE APODOUS HOI.OTHURIANS 137 



monly on the anterior part of the body, and especially on the tentacles, but in 

 only a few cases are they abundant enough to make the surface very slimy. The 

 mucus is, however, frequently abundant enough to cause sand and mud to ad- 

 here, and if such individuals are placed in alcohol the mucus is hardened and 

 the mud and sand then persist. In a few cases the calcareous deposits are 

 abundant enough to make the body-wall very firm and even brittle, while in 

 other cases the abundance of phosphatic deposits gives considerable rigidity. 

 In the latter case, however, the surface is quite smooth. As a rule, the skin is 

 much thinner and rougher in the young than in adults, and in some species the 

 young may have a thin and very rough skin, while the fully mature specimen 

 is perfectly smooth and has quite a thick skin. 



TENTACLES. The number of tentacles in the Molpadiidae is almost invaria- 

 bly 15 (Plate X, fig. 2), except in the singular genus Ceraplectana, in which 

 there are only 10 (Plate XIII, fig. 6). Of course individuals are occasionally 

 met with which have 14, and much more rarely one sees a specimen with 16. 

 In Ceraplectana there are two tentacles in each interradius, while in all other 

 cases, so far as known, there are not three in each interradius, as might be sup- 

 posed, but four in the mid-dorsal interradius, three in each of the latero-dorsal 

 interradii, three in one of the ventral interradii, and two in the other. Whether 

 it is the right or left ventral interradius which has three tentacles seems to de- 

 pend on the individual. The tentacles are always relatively short and rather 

 stout. They are simple and undivided in some species (Plate XII, fig. 22), and 

 are then usually soft and flexible, but may be firm and horny. Most commonly 

 there is a single short digit on each side, near the tip, so that there are ap- 

 parently three digits with the terminal one largest; occasionally there are two 

 or even, in very large specimens, three digits on each side, those nearest the 

 base being the smallest. In other cases the tentacles bear two pairs of digits 

 of approximately equal size, so arranged that one pair is at the tip, closely fol- 

 lowed by the other; there is then no terminal unpaired digit (Plate X, fig. 2). 

 In some cases the terminal pair is distinctly larger than the second, but more 

 commonly if one pair is larger it is the latter. The tentacles are of equal size, 

 though of course they need not always ba equally extended in a living, or equally 

 contracted in a preserved specimen. They are hollow, containing as they do the 

 tentacular canals of the water-vascular system. These canals pass directly 

 into the digits, extending to their tips. The wall of the tentacle is made up of 

 a thin cutis overlying a layer of columnar epithelium, beneath which is a thin 

 connective tissue layer, which does not normally contain calcareous deposits. 

 The cavity of the tentacle is lined with an epithelium, and between, it and the 

 connective tissue is a layer of longitudinal muscle fibers. Circular muscles are 

 probably not present, although they are said to occur in Molpadia. Exterior 

 to the muscles, on the inner side of the tentacle, is a layer of nervous tissue. 



