78 



PANAMIC DEEP SEA ECHINI. 



the case ; the plates of the actinostome are limited to an area which 

 increases but little in size. Early in the development it is covered with 

 plates which increase in number from the interior of the outer row of 

 peristome plates between it and the actinostome ; these plates increase in 

 size in all directions. 



In the Echinothurise it is easy to follow the migration of the pores 

 from the coronal ambulacral plates on to the actinal system. In a specimen 

 of Phormosoma zealandice of 21 mm. (PI. 51, fig. 1) and one of P. hispidum 

 (PI. 43, fig. 5) the third plate (counting from the teeth) is in each case in 



24 mm. 

 FlG. 12U. P. y.KAI.ANDI.E. 



15 lulu 



Fig. 130. P. hispidtjm. 



the process of passing from the corona to the actinal system, they are not 

 (|iiite free but are still held together by connecting calcareous tissue. The 

 ambulacral plates are single up to the sixth pair, Figs. 129, 130, where 

 the intercalated half plates begin and the ambulacral plates are clustered 

 in rows of three. On reaching the actinal system the ambulacral buccal 

 plates extend rapidly laterally, principally toward the interambulacral areas, 

 and overlap. 



The first pair of buccal ambulacral plates is formed on the actinal mem- 

 brane, Figs. 131, 132 ; the second pair of buccal plates is the first to become 

 detached from the ambulacral coronal plates (PI. 43, figs. 1, 3). 



In Phormosoma and Asthenosoma the ambulacral pores are placed near 

 the upper sutures (PI. 48), while in the regular Echinoidea they are gen- 

 erally near the lower suture or in the suture itself. The secondary suture, 

 such as occurs across the primordial plate of P. hispidum (PI. 43, fig. 0), is 

 the first indication of an extensive network of secondary sutures which 

 extends over the coronal plates and gives them additional flexibility (PI. 45, 

 figs. 13, 14) in older stages of growth. 



