52 EMBEYOLOGY OF THE STAEFISH. 



(p^), Avhich make a broad binding on each side of the ray, uniting with 

 the terminal plate so as to form a continuous limestone cord round the 

 edge of the Starfish. The interradial plate projects from the angle of the 

 rays towards the basal plate, spreading somewhat, to fill up the space be- 

 tween the median arm-plates. We find, in this stage (PL YIII. Fiff. 1), 

 the first dorsal water-tubes {d') ; there are five pairs, one tube on each 

 side of the ovarian plate (p^). But, as yet, no pedicellariae have appeared. 

 From the lower side, no trace of the plates of the interambulacral sys- 

 tem can be seen, beyond the spines which have formed at the extremity 

 of the ambulacra. The ambulacral pores are arranged in a single row 

 on each side of the median line, and the slender last-formed tentacles 

 are placed at the extremity of the ray, nearest to the odd ocular ten- 

 tacle ; while the tentacles nearest the mouth are quite short and stout, 

 having a large sucking disk, "and resembling, in all respects, those of the 

 adults. The separation of the different ambulacral plates is very faint, and 

 does not become well marked till a later stage. The odd ocular tentacle 

 has retained its function ; the eye-speck has increased greatly in size, as 

 well as the bulb to which it is attached, while the walls of the tentacle 

 are nearly as thin as in the younger stages (PI. VIII. Fig. 5), exhibiting 

 no trace of the formation of any sucking disk. Nearest to this are found 

 the last-formed tentacles, easily recognized by their length, and the some- 

 what less developed sucker. These and subsequent stages of the young 

 Starfish show undoubtedly that new tentacles are formed at the extremity 

 of the rays, while new portions of the upper part of the arm are formed 

 at the base ; that is, the actinal system is developed at its periphery, 

 while the abactinal system is developed at the centre. 



In young Starfishes of two years (PI. VIII. Fig. 8) the median plate 

 is longer, more closely crowded with spines ; the terminal plate being 

 loss prominent, though still distinct, while the processes from the median 

 and lateral plates are quite large. No additional dorsal water-tubes have 

 been formed since the last stage (PI. VIII. Fig. 1). When examined from 

 the oral side, the median line is becoming more strongly marked, and 

 the lateral and ambulacral spines more prominent. Tliese features give 

 to the young Starfish a more pointed appearance, and the resemblance to 

 the adult now becomes more apparent. 



In somewhat older specimens (three years old) (PI. VIII. Fig. 10), we 

 finally trace the first ajipcarance of pedicellarice (PI. VIII. Figs. 2, 3, 4, 



