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 j)resents an advance upon that condition. 

 There is an important feature in the structure of the oral pyramid of Hcqilocrinus 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 Knopfchen." 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 Haplocrimis 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, Stellericls, 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 Platycrinidse 

 and Actinocrinidse 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 ; whUe the orals of Haplocrinus, also interradial, rest 

 against the single plate discovered by Wachsmuth, which occupies a central position in 

 the summit, immediately above the peristome, as in so many other Palseocrinoids. I 

 propose to call it the " orocentral." 



In the remarkable series of specimens of AUagecrinus^ which have been obtained by 

 the collectors of the Geological Survey of Scotland, the smallest and least developed 

 individuals are in the same morjihological 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, Echinodennen des Eifler Kalkes, p. 105. 



2 Petrefacta Germanire, Tlieil. i. p. 214, tab. Ixiv. figs. 6i, 6c 



3 Quart. Jonrn. Micr. Sci., 1879, vol. xix., N. S., pp. 179-182 ; 1884, vol. xxiv. p. 14. 



* Loc. cit, p. 621. '" Ann. and Mag. Nat. Hist., 1881, ser. 5, vol. ^'ii., pp. 282-289. 



