Vol. 55.] ECHINOIBEA AND OPHIUROIDEA. 707 



star-fishes and urchins. Haeckel, however, among the latest writers 

 on the subject, refers it to the Agelacrinidae, and in doing so follows 

 Zittel/ Lang has raised several objections to the inclusion of 

 Mesites with the Echinoidea : he remarks ^ : — ' (1) It is quite uncertain 

 that the ambulacral vessel lay in this [the inner] canal, and not in the 

 outer channel ; (2) the ambulacral feet, in Echinoids, pass through 

 the ambulacral plates and not between them as in Mesites \ and 

 (3) it is not at all certain that the pores of Mesites really served for 

 the passage of ambulacral feet.' None of these objections, however, 

 apply to Palceodiscus ; and, considering the positive nature of the 

 evidence afforded by this fossil, the position of Mesites becomes a 

 matter of secondary importance, and one upon which I do not feel 

 impelled to pronounce an opinion, especially as I have had no 

 opportunity of examining actual specimens. 



Regarding the suggestions which have been offered as to the 

 connexion between function and structure in the case of urchins 

 and star-fishes, reference may be made to the ambulacral spines : 

 these, on the theory propounded, are not merely protective in 

 the usual sense, that is, against enemies, but serve as struts, 

 preventing the tube-feet from pulling the body of the animal into 

 immediate contact with the ground; and thus protect it from 

 bruising and attrition. 



It is noteworthy that in Pdlceodiscus the test has not yet acquired 

 the rigidity that characterizes the majority of Echinoids, and hence 

 at this stage of development there is an obvious advantage in the 

 possession of twofold ambulacra. As tesselation becomes more 

 compact this advantage is diminished, and the Asteroid ambulacral 

 series finally becomes functionless and disappears ; except, and the 

 exception is significant, in the case of the ambulacral auriculae : 

 these, as Johannes Miiller long ago divined, represent the otherwise 

 lost internal ambulacral plates, and they have survived because they 

 have a function to serve, namely, as processes of attachment for 

 the retractor-muscles of the buccal armature. 



EcHiNOCYSTis POMTJM, WyviUc Thomson. (Pigs. 12-14, pp. 708 & 

 709.) 



Only one out of a series of seven specimens has been crushed 

 from above downwards, all the rest are laterally compressed. The 

 primary tubercles are centrally pitted or perforate. 



Sir Wyville Thomson asserted that teeth were borne by the outer 

 ends of the alveoli ; but Prof. Duncan questioned this, and regarded 

 their presence in this position as accidental. It is difficult to 

 be certain on this point. I have repeatedly observed appearances 

 similar to those figured and described by Wyville Thomson ; 

 the spines have every appearance of being proper to the alveoli, 

 but the buccal membrane, which bears similar spines, extends so 

 close to the extremities of the alveoli, that the association with the 



^ Zittel, ' Grundziige der Palaeontologie,' 1895, p. 157. 



' Arnold Lang, ' Text-book of Comparative Anatomy,' Engl, transl. (1896) 

 pt. ii, pp. 386, 387. 



