50 THE APODOTJS HOLOTHURIANS 



anchor consists of a more or less curved shaft, lying with the convex side 

 against the plate, the posterior end (stock) frequently, if not always, con- 

 nected with the bow or posterior portion of the plate by connective tissue, 

 and the two arms, which are more or less outwardly flaring. The arms are 

 frequently toothed on their outer sides, the teeth, however, pointing, not back- 

 ward, but forward or outward. When the arms are smooth, there are often 

 several small knobs on the vertex of the anchor, where the two arms and the 

 shaft fuse. The stock is more or less expanded, and may be undivided and 

 edged with numerous fine teeth, or it may be more or less branched, the 

 branches bearing teeth. In exceptional cases the anchors bear arms at both 

 ends. Not infrequently the anchors are asymmetrical, one arm being larger 

 than the other, and occasionally a third arm occurs. The anchors of the pos- 

 terior end of the body are oftentimes longer than those of the anterior end, and 

 sometimes there are two entirely different kinds of anchors found in a single 

 species. Anchors vary in size from 0.12 to 1.1 mm. in length, those of the latter 

 size being quite easily seen with the unaided eye. They arise as straight rods 

 lying perpendicularly to the long axis of the animal, and therefore at right 

 angles to the rods giving rise to the plates. They are of considerable size be- 

 fore the plate rods appear, and already show the rudiments of the arms, which 

 extend outward at first, and later bend backward. The teeth on the arms, and 

 the branches and teeth on the stock are the last parts to appear. The number 

 of anchors and plates varies greatly, not only with their size, but also with the 

 habits and habitat of the animal. In some species there may be only a few 

 less than 25, perhaps to a square centimeter of skin, while in other cases 

 there are from 1,000 to 1,500 per sq. cm. They are often more numerous pos- 

 teriorly than anteriorly. 



The so-called wheels (Plates VII and VIII) occur in a number of genera, 

 in several forms, either scattered singly in the skin or collected in little heaps 

 or papillre. These heaps are generally large enough to be visible to the naked 

 eye, and then appear as small white spots. They are covered only by the very 

 thin epidermis and are easily rubbed off. They occur chiefly on the inter- 

 ambulacra, often in definite longitudinal rows, but are sometimes scattered 

 over the whole body. When the wheels are scattered and not in heaps, they 

 are usually quite widely separated ; but they may be crowded, and occasionally 

 so much so as to form several layers and make the body-wall firm and hard. 

 The wheels are sometimes flat or nearly so, but more commonly they are de- 

 cidedly concave, somewhat saucer-shaped, so that the margin is higher than the 

 center, though the latter may be higher than the surrounding spokes. Most com- 

 monly the wheels have 6 spokes, which are rather wide and flat, but in other 

 cases the spokes are narrower and more numerous, from 8 up to 25 in number. 

 The rim of the wheel is usually smooth on its outer margin, though some- 



