SPINES OF THE YOUNG STARFISH. 47 



of the disk, and soon reach the base of the radiating canal (PI. VI. Fiff. 7). 

 There they unite by bridging the intervening spaces, and form five tri- 

 angular openings, enclosing the tentacles, which are still at liberty, with 

 the exception of this band across the base of the radiating tubes (PI. VI. 

 Fig. 9). The additions made to this deposit of limestone take place more 

 rapidly near the bridge, where additional limestone cells are sent out, 

 enclosing at first the basal pair of tentacles, but leaving the remaining 

 five still unconfined. The next pair is then imprisoned by a similar pro- 

 cess, without, however, interfering with the terminal tentacles. Finally, 

 the last pair of tentacles is surrounded in a like manner, and all the ten- 

 tacles are now confined somewhat as we find them in the adult (PI. VI. 

 Fig. 12 ; PI. VII. Fig. 1). A row of limestone cells, extending along 

 the median line, separates the base of the suckers, while transverse bands 

 join the larger cells of adjoining spaces. It is plain that the transverse 

 bands correspond to the ambulacral plates of the adult, and that, in the 

 earlier stages, the embryo Starfish has no trace whatever of any inter- 

 ambulacral system. This mode of formation of the ambulacral system may 

 explain the absence of interambulacral plates in the Crinoids and Ophi- 

 urans. The deposit of limestone is not sufficiently transparent to allow a 

 good view of the radiating canal, or of the formation of the vesicles of 

 the tentacles. 



Formation of the Spines of the young Starfish. — We have seen that, at the 

 time of the closing of the young Starfish, the abactinal region is already 

 covered with regular rows of spines (PI. VI. Fig. 4). These spines are, 

 however, simple warts, slight protuberances, in which limestone cells are 

 formed, connecting with the general network. The cells of these spines 

 are arranged in regular tiers one above the other; the younger v^ells, 

 formed at the base, being always more numerous, and pushing up the 

 older ones. All the cells send off Y-shaped appendages, which unite, 

 forming stories (PI. VII. Figs. 3, 4, 5) of circular cells; the cells of the 

 spine near the edge do not close, but project beyond the margin, giving 

 the spines the appearance of small Gothic spires. 



The spines of the first row — viz. those immediately on the edge 

 of the rays — increase rapidlj-, curving sideways, expanding at the tip, 

 and assuming as fantastic shapes as those of Rhabdocidaris Orbygniana 

 (PI. VI. Figs. 10, 11, 12, p p). The other rows of spines, diminishing in 

 size as they approach the centre, are exactly similar to the former (/>i. 



