112 PANAMIC DEEP SEA ECHIXL 



Seen from the interior (PI. 50, fig. ^), the ambulacra! tube gives off one 

 branch to each plate, which evidently terminates in a blind sac, as no trace 

 of its pore can be seen from the exterior. 



The point of disappearance of the second small plate is shown on 

 Plate 50, fig. 4/ where the two small plates are shown at the suture of the 

 lowest plates, while in the older plates the outline of the second small plate 

 has disappeared, and its pore alone is left close to the poriferous plate, 

 gradually to move in older plates to a more central position (PL 50, fig. 3) 

 with the extension of the poriferous plate along the suture. This figure 

 also shows the tentacle of the lozenge-shaped poriferous plate. A number 

 of spheridia are shown on Plate 50, figs, i, 3 at various points of the ambu- 

 lacral area. 



The exclusion of the young abactinal ambulacral plates from the outer 

 edge of thoir respective ambulacral zones is quite unknown in the regu- 

 lar Echinidie, and forms a marked exception in Kamptosoma, where the 

 new embryonic half- plates originate along the median ambulacral line 

 and are at once pushed along the middle part of the principal ambula- 

 cral plates (PI. 50, fig. S). The large ambulacral plates are imperforate 

 (PI. 50, fig. o) w'hon seen from the exterior, when examined from the 

 interior the main ambulacral tube is seen to send a branch to each of 

 the larger primary plates, but it ends in a blind extremity and does not 

 pass through the large plates. 



On the actinnl side the fifth or sixth pair of interambulacral plates carry 

 two primary tubercles ; on the abactinal side these plates carry secondary 

 tubercles, grouped by twos, three to four to each plate, half way to the 

 ocular plate. 



The actinal system measures only 9 mm. ; it is smaller compared 

 to the size of the test than in other Echinothurids (PI. 50, fig. 7), and 

 has retained the embryonic feature of having the ten primitive bare 

 buccal plates entirely out of proportion to the size of the second row of 

 actinal plates and occujjving the greater part of the actinal system,' with 

 the exception of the narrow belt in which is placed the second row of actinal 

 plates, as well as the minute irregular elongate calcareous plates which 

 separate it from the ten original plates. 



In this species, as in older stages of Ph. hi^/ndtwi, we find a few of the 

 same irregular elongate interambulacral plates which in the Cidaridae are 



* The lower plates are the abactinal plates. " As in Ph. placenta (PI. 43, fig. 7). 



