THE APODOUS HOLOTHURIANS 51 



times it bears prominent spines, alternating with the spokes. The inner 

 margin is generally toothed, either with numerous minute teeth or with from 

 15 to 30 larger teeth which point inward, but it may be perfectly smooth. The 

 center or "hub" of the wheel is usually smooth, but is sometimes rough on 

 the outer end. Its inner end is the point of attachment for a strand of con- 

 nective tissue, the exact function of which is not known. In size the wheels 

 vary from about .30 to upward of 350/i, the diversity being very notice- 

 able oftentimes in a single individual. The various kinds of wheels arise first 

 either as discs or saucer-like plates, with fluted edges, or as star-shaped bodies 

 (Plate VIII, figs. 9-12). The spokes arise either from the projections on the 

 edge of the plate or from the rays of the star. As they increase in length, they 

 expand at the outer end, and the expanding branches meet and fuse to form 

 the rim of the wheel. 



The curved rods occur most commonly as supporting rods in the tentacles 

 (Plates IV, V, and VII), but are sometimes found scattered in the body-wall 

 itself. In the former case they are usually more or less branched, toothed, or 

 perforated at each end, though in some cases they are smooth and solid. They 

 show great diversity in size and shape, ranging from 20 to 200 m. Their 

 long axes are generally parallel to the long axis of the tentacle or to that of 

 the digit in which they lie, but they are sometimes collected more or less irreg- 

 ularly near the base of the tentacles, especially on the inner side. The rods 

 which lie scattered in the body-wall may be only very small elongated parti- 

 cles, with smooth, rounded ends (Plate IV, fig. 14), or they may be more or 

 less curved, having the ends somewhat enlarged and often toothed (Plate VII, 

 figs. 4, 7, and 12) ; or the form may be that of a hook or bracket or an 

 elongated letter S (Plato VII, figs. 5, 6, 9, 10, and 11). The latter are called 

 sigmoid deposits. In such deposits the curves of the opposite ends may lie 

 in the same plane or in planes at right angles to each other. The rods and 

 sigmoid deposits of the body-wall are irregularly scattered, though the latter 

 generally lie at right angles to the longitudinal axis of the animal. The rods 

 range from 20 to 100 /i in leno-th, while the sigmoid deposits are larger, 

 usually measuring between 100 and 200 m. The sigmoid deposits arise as 

 simple, smooth, curved rods, which gradually increase in size and curvature 

 until the perfect form is assumed. 



The so-called miliary granules (Plate IV, figs. 16 and 21) are very small 

 particles of lime, sometimes smooth and rounded, sometimes cross- or star- 

 shaped, and sometimes branched and irregular, which occur not only in the 

 connective tissue of the body-wall, but often in the underlying muscles also, 

 both circular and longitudinal. Generally they occur scattered more or less 

 uniformly in the body-wall, but are sometimes confined to the radii. They are 

 often collected in small heaps or patches, which appear to the naked eye as 



