76 PANAMIC DEEP SEA ECHINI. 



same species of 9 nun., where there were fewer anal plates (PI. 43, fig. 4). 

 The position of the anus, which is central in these young specimens, seems 

 to trend to one side, as will be noticed on examining the figures of the apical 

 system of P. hispidum (Pis. 39, 40). In Astkenosoma coriaceurn it has 

 retained its central position (PI. 52, fig. /). 



A similar movement in the anal system of Salenia^ is suggested by the 

 gradual greater encroachment, due to age, of the right posterior ocular 

 plate upon the anal system (PI. 16, figs. .', /,, <;,) and in Acrosalenia (PI. 22, 

 figs. 5, 6, 7, 8). 



When about 30 mm. in diameter the genital and ocular plates, which 

 had thus far formed a closed ring, begin to separate, the madreporic genital 

 remaining longer in contact with the odd ocular than the other genitals 

 do with their correlated oculars (Pis. 39, 40). 



In Phormosoma (PI. 40, fig. ;.') and Asthenosoma (PI. 51, fig. 6) the 

 genital pores are rarely in the genital plates ; they are usually situated in 

 the triangular extension of the dermal tissue which separates the abactinal 

 region of the interambulacral zones. The genital pores open in a small 

 conical plate isolated from the components into which the genital plates 

 have become broken up (PI. 39). In most of the species of Phormosoma 

 the anal opening is surrounded by a series of elongated conical plates 

 radiating from it. 



In the Echinothurioe the proportions between the actinal system and the 

 diameter of the test vary with age. In smaller specimens of 15 mm. and 34 

 mm., Fig. 128, the actinal system is about one third that of the test ; in speci- 

 mens of 53 and 75 it is less than a third ; and in specimens of 120 to 200 

 mm. the actinal system is only a fifth of the diameter of the test. 



The interambulacral primordial plate is very prominent in the young of 

 Phormosoma (PI. 43, figs. /, 8, .5), though very different in shape in the 

 different species, it is nearly square in P. hispidum (PI. 43, fig. 5), while 

 it is very elongated in P. placenta (PI. 43, figs. 1, o). It is deeply indented 

 in Asthenosoma zealandim (PL 51, fig. 1), and quite asymmetrical in A. pel- 

 lueidum (PI. 51, fig. J), and very large, somewhat polygonal in Kamptosoma 

 (PI. 50, fig. 1). In a specimen of P. hispidum, of 34 mm., the primordial 

 plates are split in two by a vertical suture (PI. 43, fig. r,). 



On the buccal plates the first iniliaries to appear are those adjoining the 

 sutures of contiguous ambulacra. The growth of the ambulacra! plates on 

 the actinal system does not in the Echinothurise any more than in other 



