54 PAXAMIC DEEP SEA ECHIXl. 



In young Salenioe there are also traces of the single primitive interam- 

 bulacral plates, as in the small S. vaiispina of 1.9 mm. (PI. 21, fig. i). 



On several occasions I attempted to obtain information regarding the 

 succession of the coronal plates, but have been unsuccessful both with 

 Strongylocentrotus ^ and Arbacia.^ For many years at Newport the pluteus 

 of Arbacia was raised until well after the formation of the Echinid, and 

 although one stage of the young Arbacia (PI. 54, fig. l) attained 1.5 mm. in 

 diameter, I was unable even in that specimen to define the coronal plates. 

 It is only in a young specimen of 3.6 mm. that the primary tubercles are 

 fully developed, the arrangement of the coronal plates easily made out, 

 and the plates of the apical system sharply defined (Pis. 53, fig. 9; 54, fig. 

 2), though in the younger specimen of 1.5 mm. the number of the tentacles 

 and of the primary spines would indicate the probable number of the ambu- 

 lacral as well as interambulacral plates. 



Arbacia, as one of the few regular Echinid genera in which the single 

 primordial interambulacral plates are not resorbed (PI. 54, figs. 5, G), natu- 

 rally formed an interesting genus in which to trace the development of the 

 coronal plates. Unfortunately the existence in very early stages of huge 

 pigment spots renders the detection of minute calcareous plates most difficult. 

 In a young specimen of Arbacia onl}' 0.4 mm. in diameter exclusive of the 

 spines, and 0.8 mm. including them (PI. 53, figs, l-'j), there were no traces 

 of coronal plates. The larval envelopes still surround the whole animal. 

 There are fifteen large club-shaped radioles dotted with pigment; the}' are 

 arranged in sets of three, and between them are placed the five ambulacral 

 suckers, one of which, the terminal one, is much larger than the others and 

 provided with a powerful disk (PI. 53, figs. 2-6). It bends toward the acti- 

 nal system, as is the case also with other larval Echini (Strongylocentrotus 

 and Goniocidaris). The abactinal side is greatly swollen (PI. 53, fig. J), 

 with five large plates, presumably the oculars, on the abactinal side of the 

 large odd terminal sucker. Above the ambitus four diminutive spines have 

 made their appearance. Seen from the actinal side (PI. 53, fig. 5) the five 

 actinal plates which have appeared may be the first trace of the teeth. 



In the stage of 0.4 mm. the five ambulacral suckers correspond to the 

 five embryonic folds of the water system, showing that in the earliest stages 

 of Arbacia as in other young Echini there can be only four ambulacral 



» Kmbiyol. of Eohinod., Mem. \m. Acad. 1864, IX, T\. 1 ; Revision of the Ecbiiii, p. 709 (PI. 

 IX, fig. 1 ; IM. X, figs. 1-4). 



2 Rovision of the Echiui, p. 734, figs, fiS, 60. 



