114 Papers from the Marine Biological Laboratory at Tortugas. 



relatively but actually smaller than when younger. Evidently a remarkable 

 process of resorption is going on. The extent of this resorption is well shown 

 by the following table : 



Besides the change in size, another change is going on in this plate, 

 during the growth of the disk that is no less remarkable. In the youngest 

 specimen examined the plate is a smooth, thin sheet of lime, entirely 

 unperforated except for a number of minute holes around the margin. As 

 resorption goes on these holes encroach more and more on the plate, not by 

 increase of size so much as by increase of number. When the plate is only 

 0.25 mm. across, the unperforated area has practically disappeared, and after 

 that stage is reached it is no longer practicable to distinguish the plate from 

 the numerous other small plates at the center of the disk. It can not be said 

 whether the plate is resorbed entirely or whether a minute portion persists. 

 It will be noticed that this plate is utterly unlike anything hitherto reported 

 among echinoderms. Instead of beginning as a triradiate spicule, and 

 steadily increasing in size by dichotomous branching and fusing of the 

 branches, as an echinoderm plate typically does, this plate starts as a large, 

 homogeneous disk which by a process of resorption gradually becomes like 

 its associated plates in both size and structure. In the present state of our 

 knowledge it is futile to speculate on the significance or homologies of this 

 peculiar plate. 



Very early in the formation of the disk-covering, but not while the 

 primary plates (excepting indeed the central) are still conspicuous, there 

 begin to arise (in association with the plates) vertical rods trifurcate at 

 their free end. These trifurcate spinelets are usually found one to a plate, 

 but on the radial shields there may be a number, arranged in series of 3 

 or 4, parallel to the radial margin of each plate. At first there are no 

 spinelets associated with the central plate, but when the disk^ is about 

 4 mm. across there may be as many as 5 spinelets upon it. During youth 

 the spinelets are all of approximately uniform height and terminate in 3 

 sharp teeth of moderate length; but as maturity is reached some of these 

 spinelets grow out into spines, not unlike the arm-spines in length and 

 appearance. They are two or three times as long as the spinelets. The 

 number and distribution of these disk-spines is very diversified and the 

 appearance of the disk is greatly modified by their appearance. As a rule, 

 it may be assumed that the presence of these spines in abundance is indica- 

 tive of age and their absence is indicative of youth, but the rule is by no 

 means unvarying. The number of spines and spinelets on the radial shields 

 is another variable factor in the appearance of the species. When the 



