158 THE VOYAGE OF H.M.S. CHALLENGER. 
If, as Zittel supposes, the ambulacra of the arms were continued past the radial 
openings and up the steep sides of the oral pyramid, which is closed at the apex, how did 
they reach the peristome underneath? Did not food-grooves and the subjacent 
ambulacral structures pass in beneath the oral pyramid at the radial openings, so as to 
reach the mouth and the vascular and nervous rings around it, just as they must have 
done beneath the dome of Actinocrinus? It appears to me that Haplocrinus is 
permanently in the condition of a Pentacrinoid larva with a closed tentacular vestibule ; 
though in other respects, such as the attachment of the brachial plates by muscles’ and 
ligaments above a perforated articular ridge, it presents an advance upon that condition. 
There is an important feature in the structure of the oral pyramid of Haplocrinus which 
has long escaped notice. Goldfuss? long ago described the furrows which mark the 
sutures between the oral plates, and went on to say “wo diese im Scheitelpuncte 
zusammenstossen, steht ein rundes Knépfchen.” This is quite clearly shown in his figures, 
but has attracted no attention from subsequent workers. Lately, however, it has been 
discovered by Mr. Wachsmuth that there is really a small but distinct plate in this 
position, occupying the central portion of the summit of the oral pyramid. I am indebted 
to him both for the opportunity of verifying this observation by examination of his 
specimen of Haplocrinus mespiliformis, and for permission to make use of it in this 
chapter. This plate is one of considerable importance in its morphological relations. In 
accordance with the views which I have expressed elsewhere,’ I believe it to be the 
representative on the actinal side, or left larval antimer, of the dorsocentral plate which 
is developed in the centre of the right antimer or abactinal side of Urchins, Stellerids, and 
Crinoids. I have reason to think that this belief has been adopted by Mr. Wachsmuth 
not only so far as regards Haplocrinus and Symbathocrinus, but also for the Platycrinidee 
and <Actinocrinidz as well. The orals, as shown by Goette,* are the actinal repre- 
sentatives of the basals, being developed spirally around the left vaso-peritoneal tube ; 
while the basals, like the so-called genitals of Urchins and Starfishes, appear in a spiral 
around the right peritoneal tube. They are disposed interradially, and rest in most cases 
directly against the dorsocentral; while the orals of Haplocrinus, also interradial, rest 
against the single plate discovered by Wachsmuth, which oceupies a central position in 
the summit, immediately above the peristome, as in so many other Paleocrinoids. I 
propose to call it the “ orocentral.” 
In the remarkable series of specimens of Allagecrinus’ which have been obtained by 
the collectors of the Geological Survey of Scotland, the smallest and least developed 
individuals are in the same morphological condition as Haplocrinus, or perhaps even in 
a more primitive one. The calyx is covered by a round dome of oral plates, which rests 
1 Schultze, Echinodermen des Hifler Kalkes, p. 105. 
2 Petrefacta Germaniz, Theil. i. p. 214, tab. lxiv. figs. 6, 6c. 
$ Quart. Journ. Mier. Sct., 1879, vol. xix., N. S., pp. 179-182; 1884, vol. xxiv. p. 14. 
4 Loc, cit., p. 621. 5 Ann. and Mag. Nat. Hist., 1881, ser. 5, vol. vii., pp. 282-289. 
ca 
See, 
