46 SVEN LOVEN, ECHINOLOGICA. 



and wlien they imbricate it is aborally. We novv can add 

 that in tlie Cidaridee tliey alford tlie anricles, while in the 

 Ectobranchiates these are borne by the ambnlacra. In the re- 

 cent Ectobranchiates, of which many are neonomous, this in- 

 herent dissimilitnde, while maintaining its full validit3^ is ob- 

 scured in the outward aspect by the ambulacra increasing in 

 lireadth and forniing Avith the interradia an even set, as also 

 by the spinary system tending to equality and uniformity all 

 över the perisome, covering it witli verrncte and spines similar 

 in form and approximating in size. This appearance of con- 

 formity prevalent among the numerous EchinidcE, is less 

 marked in the Arbacidse and in the Saleniidas, of at least oolitic 

 <irigin; it is effaced in the Echinothnrite and Diadematidte, 

 also of remote beginning, which present great contrasts be- 

 tween the two systems, narrow ambnlacra of aberrant forms 

 and with peculiar spines, and in certain genera imbricated 

 adorally, while the interradia are imbricated aborally. And in 

 the Cidaridér, the oldest of recent Echinoidea, the difference 

 beconies obvious. Their ancestors lived as Perischoechinoids 

 in the Silnrian seas, once the home also of Cvstoids, in some 

 of which, as in Glyptosphsera and Protocrinns, the ambulacra 

 formed part of the perisome, while in others, as in Callocy- 

 stis, they were attached solely by their first adoral plates but 

 for the rest free, movable, reclining on the perisome;' and in 

 numerous others »arms> were directed upwards, arising round the 

 oral region. Perhaps that, when the dental apparatus at first 

 originated in some remote ancestor of the antique Perischoechi- 

 nidtie, the ambulacra were yet free and movable, wholly ex- 

 tern al, and nowise internal parts, and thus the interradia 

 alone liad to afford, out of their own snbstance, the supports 

 required for the retractor muscles, and that, when the pyra- 

 mids came into existence in the Ectobranchiates, the ambu- 

 lacra were already fixed, fettered limbs, strong enough to su- 

 stain their auricles. 



From the structure of the masticatory parts in the Cidaridte 

 and the Ectobranchiates, their mode of action is easily inferred. 

 The Echinoids are slow animals. In the Regularia the defence is 

 left to the bristling spines and the poisonous forcipes, the opera- 

 tion of catching free and active prey to the agility of tlie numer- 

 ous, flexible and contractile pedicels armed with powerful sucking 



' See the autlior's memoir on Pourtalesia, p. 10, 57. 



