Growth-changes in Brittle Stars. 99 



centrifugally, while the covering of the regenerated disk arises centripetally. 

 As division appears to take place repeatedly, by the time savignyi is full 

 grown there is not the least trace of the original primary plates nor of any 

 very definite arrangement in the numerous small plates covering the center 

 of the disk. The radial shields are always conspicuous and cover a large 

 proportion of the disk. Soon after the disk is a millimeter across, spinelets 

 arise in connection with the marginal disk-plates. These spinelets are not 

 outgrowths of the plates, but arise from separate centers of calcification in 

 the tissue covering their outer surface. Not often is more than one associ- 

 ated with a single plate, and many plates, especially near the center of the 

 disk, have no spinelets. When fully formed the spinelets are attached to 

 the underlying plates, but no actual union takes place and the spinelets are 

 always movable to some extent. The number of these spinelets varies in 

 different individuals, but they seem to be relatively most numerous in young 

 specimens. Adult, dry specimens commonly show them plainly, but they 

 are often difificult to detect in fresh or alcoholic adults. 



MOUTH-PARTS. 



As has been stated above, the plane of division in schizogony does not 

 cut through the jaw, but passes to one side, so that the resulting animals 

 may have either 2, 3, or 4 jaws. The formation of new jaws takes place 

 rapidly, but follows of course the formation of the new rays. Consequently 

 the same factors which control the new-ray formation govern the develop- 

 ment of new jaws. In case there are, after fission, only 2 jaws on one result- 

 ing half (pi. I, fig. 3), the outer side of each of the lateral rays must assist 

 in the formation of new jaws, while in case there are 4 jaws each of the 

 new-formed lateral rays will give rise to jaw-elements only on that side 

 which is next the youngest ray. The formation of the jaws takes place as 

 already described by Ludwig (1881) for Amphipholis and I have nothing of 

 importance to add to his account. Ludwig has not discussed, however, the 

 formation of teeth and oral papillae, nor did Fewkes (1887) pay any special 

 attention to these points. I found it possible to trace the development of 

 teeth and oral papillse in both Ophiactis and Amphipholis, but as my material 

 was more satisfactory in the latter genus, the account may for the present 

 be deferred. I need only say here that there is no difference between the 

 two genera save in the number and form of the teeth and papillae. As is 

 well known, the torus (the plate which bears the teeth) forms at the tip 

 of the jaw where the two jaw-plates (adambulacrals) meet. It is at first 

 about as wide as high, but grows rapidly vertically and soon is twice as 

 high as wide or even more. As will be shown under Amphipholis, the oldest 

 tooth is at the top of the torus, the new ones forming below it. As soon as 

 a tooth is well formed, the part of the torus against which it rests is resorbed 

 and thus a socket is formed for it. The process of resorption goes so far 

 that the torus becomes perforated (pi. i, fig. 5). On an adult jaw of 

 Ophiactis savignyi there are 5 or 6 teeth of approximately uniform size (pi. i 



