472 Mr. D. W. Devanesen. Development of the Calcareous 



Each compass can be traced to a pair of tri -radiate spicules placed, not side by 

 side, but in a radial line one behind the other (Plate 13, fig. 13, I.C. and O.C.). 

 One may, therefore, speak of an outer spicule and an inner spicule with reference 

 to the central axis of the lantern. Nor are these two spicules deposited at 

 the same time as are those of the epiphyses, for instance ; the inner first and 

 the outer next is the rule. Further, these two spicules, though in the same 

 line, occupy different positions and levels in relation to the rotula. The outer 

 spicule is situated almost behind the rotula and almost at the same level as 

 the latter, if not below it, whereas the inner spicule is placed just above the 

 rotula. This fact accoivnts for the bent condition of the compasses in 

 the adult. 



These spicules differ somewhat in the manner of their growth, not only from 

 the other members of the lantern, but also between themselves. One of the 

 three rays of the inner spicule is absorbed gradually (Plate 12, fig. 3, I.C.) and 

 the remaining two rays stretch end to end, forming a straight rod, more or 

 less, directed radially. The free-branching and anastomosing of the original 

 rays of the rudiment, found in the development of the rotula for example, 

 does not obtain here. The next step in their growth consists in an increase 

 in thickness by the appearance and coalescence of branches at the two ends 

 of the rod ; the middle part of the rod appears to grow in thickness all the 

 same, though one does not see as many offshoots there. 



The outer spicule has all the three original rays well developed ; neverthe- 

 less they do not branch. Its growth is confined to its size. The three ends 

 of the spicule appear in a late stage thick and fenestrated, but the body of the 

 spicule is devoid of meshes (Plate 15, fig 6, COM.). The tri-radiate shape is 

 maintained intact and the ray directed centralwards meets the inner piece and 

 forms a suture with it, thus constituting the adult compass. It is at this 

 suture that the elevator muscles of the compasses are attached, some of the 

 fibres to the hinder end of the inner piece and others to the proximal end 

 of the outer. This suture between the two pieces is so complete in the 

 adult echinoids, both extinct and living, that its double nature has been 

 scarcely ever surmised except in one species Stvongylocentrotus drobachiensis 

 (Jackson, E. T., p. 179). 



The compasses are the only set of ossicles of the lantern which are absent 

 in the " echinus-rudiment " of the larva, but appear after the metamorphosis. 



7. The Teeth. 



At the very outset it may be stated that the tooth is, in the lantern 

 organisation, an element with a unique structure and a peculiar develop- 

 ment. 



