FORMATIOX OF THE SPINES. 39 



Figs. 10, 12) ; wliile, in the other ca.se, a full view of the abactinal pen- 

 tagon (PI. III. Fifj. 10; PI. IV. Fifi. 4; PI. V. Fifis. 5, 11) is obtained, and 

 the arrangement of the different rods forming the plates of the limestone 

 network is distinctly seen. A view of the larva from the dorsal side 

 (PI. III. Fif). 11; PI. VII. FUj, 8; PI. V. Fvih. 8, 9, 13) shows the abac- 

 tinal system extending in such a way as to surround the stomach en- 

 tirely on one side, while the tentacular pentagon covers it on the oppo- 

 site side. This attitude gives us the position of the lobes {r"i-r"l), the 

 future rays of the Starfish, next to the water-pore (r'i', r"^), while a view 

 from the oral side (PI. V. Figs. 4, 14) indicates the trend of the lobes 

 on the opposite extremity of the spiral of the abactinal S3^stem {r"i, r"). 



[Metschnikoff was able to trace most satisfactorily the development of 

 an Ophiuran in which the formation of the abactinal system was shown 

 to be identical in every respect with that here given of the Starfish. In 

 the Ophiuran of which I had previously * traced the growth the forma- 

 tion of the plates of the disk could not be seen. These I only traced 

 at that time in one of the viviparous species.] 



Formation of the Ra>/s of the future Starfish. — The plates of the abactinal 

 system early reach a condition when the changes they undergo are merely 

 quantitative, and the only modifications affecting the appearance of the 

 Starfish take place on the edge of the disk. A depression is formed in 

 the middle of the convexity of the lobes of the abactinal area ; this is 

 soon followed by two other depressions in the middle of the small arcs 

 thus formed, dividing each lobe of the pentagon into four smaller lobes ; 

 at the same time the indentations between the original sides of the pen- 

 tagon have grown much deeper, separating these five lobes in a very 

 marked manner. We can now no longer mistake the true character of 

 the lobes ; they are the five rays of the Starfish, but as the actinal and 

 abactinal regions are not yet fitted together, as in the ad'dt (PI. V. Figs. 

 10, 11, r'i'-r'5'; PI. IV. Fig. 4), they represent only the dorsal sides of the 

 rays. A glance at Fig. 9 of the same Plate (PI. V.), seen from the dorsal 

 side, will show how far the suckers {t) are removed from the abactinal 

 portion of the arm which is to protect them. The position of the water- 

 pore {b) is immediately on the edge of the disk, at the extremity cf the 

 dorsal end of the pentagon (PL V. Figs. 10, 13, h). 



* Mem. Am. Acad. 18G4. 



