42 EMBRYOLOGY OF THE STARFISH. Past I. 



so as to form a continuous limestone chord 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 between the median arm-plates. We find, in this 

 stage (PL VIII. Fig. 1), the first dorsal water-tubes (d') ; there are five pairs, one 

 tube on each side of the ovarian plate (p c ). But, as yet, no pedicellarise have ap- 

 peared. 



Prom the lower side, no trace of the plates of the interambulacral system 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 tentacle ; 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 

 (PL 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 somewhat 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 (PL VIII. Fig. 8), the median plate is longer, 

 more closely crowded with spines ; the terminal plate being less 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 (PL 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. These 

 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), (PL VIII. Fig. 10), we finally 

 trace the first appearance of pedicellarias (PL VIII. Figs. 2, 3, 4, p', p"), the dorsal 

 tubes (PL VIII. Fig. 10, d" d") are found arranged in greater number along certain 

 portions of the' rays ; while the median and lateral plates have increased so much in 

 size that the terminal plate has lost entirely the preponderance which it had in 

 younger stages, and the extremity of the arm actually assumes a rounded outline. 

 The dorsal tubes (d w ) are found numerous on both sides of the median arm-plate, 

 and along the edge of the oral lateral plates (d"), diminishing somewhat in size as 



