ORAL AND APICAL SYSTEMS Of THE ECHINODERMS. 189 



far as Spharonites is concerned, this being the type taken 

 by Loven for comparison with the other Echinoderms. 

 Wyville Thomson, indeed, regards these forms as " Crinoids, 

 or a parallel group ;" and 1 imagine their ventral surface to 

 be in every respect homologous ivith that of our modern 

 Crinoids, more especially of Actinometra. In this genus 

 the ambulacra form an open horseshoe-shaped curve, very 

 much as in SpJmronites. The mouth is placed in the middle of 

 this curve, but it is often extremely small and inconspicuous, 

 being merely a short and narrow slit in tlie peristomial 

 area. 



I have often experienced great difficulty in finding it even 

 in spirit specimens, in which the perisome was quite bare ; 

 while in dry specimens of such species as Act. Solaris, in 

 which the anambulacral plating is often very completely de- 

 veloped, I should, like Miiller,! have altogether failed to find 

 it, had I not known with tolerable certainty, from other 

 considerations, where to look for it. I imagine the Cystids 

 with calycine ambulacra to have resembled, in this respect, 

 such recent Crinoids as Hyocrinus, in which there is an ex- 

 tensive anambulacral plating on the disc and minute plates 

 in the marginal leaflets at the sides of the ambulacra. We 

 find precisely the same condition in the so-called recent 

 Cystid, Hyponome Sarsii, in which the ambulacra are fringed 

 and overlapped by marginal scales, while the remainder of 

 the ventral surface " is clothed with rather small, thickset, 

 irregular scales /' and it aj)pears to me that Wyville Thom- 

 son- is right in regarding Hypoiiome as a true Crinoid. 

 According to Loveu^ the " absence of any indication of a 

 calyx" tells strongly against this view, but I believe that 

 Hyponome is merely the disc of a Crinoid, which has fallen 

 out of its calyx, and that indications of its attachment to a 

 skeleton are seen in the " five broad dichotomous rays on the 

 dorsal surface."^ 



Thus then I regard the vault of the Cystids (at any rate 

 of those with open ambulacra), as quite distinct from the 

 vault of the Palao crinoids, but as homologous with the 

 ventral perisome of our recent Crinoids. This is frequently 

 covered with an extensive anambulacral plating, which is 

 perforated by the small water-pores. In Pentacrinus most 

 of the anambulacral plates are perforated in this way, the 



' " Ueber die Gattung Comatula, Lam. und Hire Arten," ' Abhandl. der 

 Berlin Akad.,' 1847, p. 245, 



^ ' Nature/ loc. cit., p. 497. 



•^ " On Hyponome Sarsii, a recent Cystidean," ' Canadian Naturalist,' 

 1869, p. 266. 



^ See Addenda, No. 4, on p. 205. 

 VOL. XIX.-— NEW SER. N 



