300 PROCEEDINGS OF THE ACADEMY OF [1S97. 



some of a crimson color and these modify to a considerable extent 

 the general color. 



In adults there are from 71-80 gills. In the young there are 

 56 arranged as in the adult. Anteriorly they shade off into min- 

 ute papillae not visible to the naked eye. 



Full grown Cryptoehiton are covered on the dorsal surface with 

 groups or tufts of calcareous spines so closely crowded together that 

 the mantle is almost or entirely concealed. In the young the groups 

 are smaller and much more scattered (Plate VIII, fig. 1) ; in 

 addition are multitudes of small crimson spines on both the ventral 

 and dorsal aspects of the mantle. Reincke, and especially Blumrich 2 

 have worked out the development of the spine in several species of 

 Chiton. It forms above and outside the epithelial cell, from which 

 it develops, and as the spine increases in length, its base with its 

 underlying cell becomes pushed into the mantle and a hollow is thus 

 produced. Many spines forming these hollows occur in such posi- 

 tions as to produce a circular groove and the small area of epithelial 

 cells surrounded by this groove becomes a papilla. In young Cryp- 

 toehiton the entire mantle surface is thrown into these papillae 

 which become clearly outlined by the spines which form in the 

 channel about them (Plate VIII, fig. 4). These small spines not 

 collected into tufts are elliptical in cross section and the calcareous 

 portion imbedded in an organic basis is made up of four pieces. 

 The arrangement of these pieces is similar to that produced by lon- 

 gitudinally splitting into fourths a cone of small base and high 

 altitude. 



In the tufts of larger spines there are always to be seen a number 

 of short bright crimson ones from which others generally color- 

 less seem to radiate. These are not centrally located, but ex- 

 centrically on the side of the tuft next to the median line. A sec- 

 tion through either a large or small group shows a comparatively 

 deep depression in the mantle from which these crimson spines pro- 

 ject. In the epithelium of the depression, on the side next to the 

 mid-line, new spines form in considerable numbers. As others devel- 

 op the preceding ones are pushed toward the outside or away from 

 the median line (Plate VIII, fig. 5). They are at first colorless but 

 when about half formed show a crimson tint, which gradually dis- 

 appears by the time they are pushed out of the cavity. These spines 



2 Zeit. fiir wiss. Zool. Bd. LIT, 1891. 



