714 PEOF. W. J. SOLLAS ON SILUKIAN" [NoV. 1B99, 



interesting feature not met with in modern star-fishes or Ophiuroids : 

 it is closely paralleled, however, in the Ophiocistia. Unfortunately 

 we know very little of the anatomy of the buccal armature of this 

 group, but that it was practically free from the remainder of the test 

 seems to be demonstrated by the specimens of Euthmnon and 

 Eucladia in the Oxford University Museum. 



Summarizing the foregoing conclusions, I regard the lantern of 

 Aristotle as homoplastic with the buccal armature of star-fishes ; the 

 pyramids are the modified first adambulacral plates ; the epiphyses 

 have arisen from the first ambulacral plates of the Echinoid series : 

 and the teeth represent the odontophore, which has acquired a 

 persistent root ; the radius and rotula remain problematical. 



The enquiry into which the characters of Pcdceodiscus have led 

 us may be fitly concluded by an endeavour to picture to ourselves 

 the manner in which a transition may have been eflPected from the 

 Asteroid to the Echinoid type. The divergence probably commenced 

 with change of character in the buccal apparatus. As this became 

 increasingly efficient in the urchin, as compared with the star- 

 fish, so did the power of obtaining food : in correlation with the 

 increased alimentation stand the greater size of the body and the 

 length of the large coiled intestine. With the development of the 

 body, the incorporation of the arms was associated : by no means 

 an unique phenomenon, since other instances are presented, by 

 AstropMura among modern Ophiuroids, and by the Ophiocistia among 

 their extinct representatives as well as by some star-fishes. On the 

 final absorption of the arms there would be left an apical region 

 devoid of ambulacra, near which these organs would terminate ; 

 the position of the anus might readily fluctuate with the growth of 

 the intestine, and the madreporite would also be subject to wander- 

 ings as the body underwent changes in size and form. In Ecliino- 

 cystis we meet with an organism very similar to that which hypo- 

 thesis would lead us to expect : the ends of the ambulacra, the 

 madreporite and the anus, instead of being associated as in regular 

 urchins, are here free and independent. The apical disc has not 

 yet appeared, but may well have arisen, at least in part, from the 

 valvular plates of the anal pyramid, as Dr. Gregory has already 

 suggested. 



Whether Echinoids be direct descendants of Asteroids or not, 

 there can be little doubt that the two groups are closely connected. 

 Their most distinctive common mark is the possession of tube-feet, 

 and there would be considerable convenience in uniting them and 

 their extinct predecessors into a single group, which might be 

 termed the Helkopoda. Brittle-stars, which appear to have 

 diverged from star-fishes at about the same time as Echinoids, retain 

 greater primitiveness of character, but have become highly specialized 

 in their mode of progression : the extinct and existing forms might 

 be united under a new name, though less inconvenience will probably 

 result from retaining the term Ophiuroidea and extending its 

 meaning. Both Helkopoda and Ophiuroidea are strongly con- 



