MUSEUM OF COMPARATIVE ZOOLOGY. 133 



nous avons mentionnee." On page 132 he writes: "On admet que 

 ces pieces fuurchues sout formees par la division d'un ossicule discoide 

 sur la ligne mediane, et la deviation de chacune des deux moities, 

 jusqu'a la rencontre de la moitie correspondante du disque voisin h. 

 laquelle elle se soude." In these descriptions it seems probable that 

 Apostolides refers to the fii'st pair of adambulacral and possibly the 

 second pair also. It can hardly be supposed that he refers to the first 

 pair of ambulacral, which are later described by me as the spoon-shaped 

 plates after Ludwig, since these never have a V-shape. Whether he 

 refers in his description to the first ambulacral or the first adambulacral 

 (with the second), it may be borne in mind that neither has been formed 

 by a division of a " discoid " ossicle, nor are they ever five in number. 

 The members of the five pai7-s of adambulacral as well as of ambulacral 

 originate as ten separate calcifications. In the youngest condition (a 

 stage a little younger than Fig. 1 7) of the growth in which they were 

 observed the first pair of adambulacral plates were formed although the 

 terminals were very small. I am therefore led to suppose that the first 

 pair of adambulacrals appear before the terminals. In this embryo 

 (Fig. 17) the pentagonal form is but obscurely indicated. The first pair 

 of adambulacral are portions of the "maxillae" of MetschnikoflTs figure 

 (PI. IV. Fig. 18, cc). 



The first adambulacral plates originate in pairs about the mouth (Fig. 

 15). In early conditions they lie more in those axes which later become 

 the interradials, although they arise before the young has passed into 

 conditions in which either radii or interradii can be definitely recog- 

 nized. In Ludwig's Fig. 23 they are smaller than the terminals, but in 

 some of my preparations they are larger. 



In a later stage (Fig. 17), in which the rays have begun to push out, 

 the first pair of adambulacrals assume a more or less irregular crescentic 

 form, with concave edges turned to the radii. By a continued growth the 

 adradial edge of these plates begins to approximate so that plates of 

 adjacent pairs approach each other. This approximation is confined to 

 about half their interradial border. The adradial half of the rude cres- 

 cent extends to thfe mouth opening. By the approach of two of these 

 first adambulacral plates (ad^) which lie in different pairs, a V-shape is 

 given to the combination. The plates however are free along the line 

 of the interradii. The double network of these plates anpears as in the 

 radialia (Fig. 18, rp). 



Of the homology of these plates with the adambulacral there seems no 

 doubt. I believe also that Ludwig's comparison of them to the side 



