REPORT ON THE CRINOIDEA. 71 



cliately surrounding the peristome and covering it more or less completely (PI. III. 

 fig. 2 ; PI. Vc. fig. 6, 0; PL VI. figs. 3, 4 ; PI. X. figs. 7, 20 ; PI. LVI. fig. 5). 

 Their rudiments appear in the free-swimming larva simultaneously with those of the 

 basals, which are developed spirally round the right peritoneal tube ; while the orals 

 appear in a similar spiral around the left one. 



The skeleton is at first limited entirely to these two rings of plates, the edges of 

 which meet around the equator of the growing cup, though they ultimately become 

 separated by the appearance of the radials between them. 



At the base of the closed pyramid formed by the oral plates is the upper portion of 

 the larval body, in the centre of which the opening of the mouth is formed. The rest of 

 the space above the circular lip and beneath the oral pyramid is occupied by the 

 tentacular vestibule. This, according to Goette,' is derived from the left peritoneal tulje, 

 and contains the fifteen first formed tentacles which are borne on the water-vasculai- 

 ring. At a certain period of development the five valves of this oi'al pyramid graduall}- 

 separate so as to open the mouth to the exterior and allow of the protrusion of the 

 tentacles ; while the floor of the original tentacular vestibule, with the mouth in its 

 centre, becomes the peristome of the growing Crinoid. Five of the tentacles correspond 

 to the intervals between the oral valves ; and a conical projection, the commencement of 

 a ray, appears at the base of each of them. The growing rays are supported by the first 

 radial plates, which appear in the rapidly expanding equatorial portion of the body, i.e., 

 the band of perisome between the upper edges of the basals and the lower edges of 

 the orals. As the rays grow the second radials appear between the bases of the orals, 

 and the equatorial band continues to increase in diameter. But the orals maintain their 

 original position round the mouth, so that they become completely separated from the 

 basals and radials by the equatorial perisome and are relatively carried inwards, while the 

 second radials project somewhat outwards. The diameter of the oral circlet continuall}' 

 decreases in proportion to that of the disk, which enlarges rapidly as new arm-joints are 

 added in succession. The orals are thus left as a circlet of five separate plates protecting 

 the peristome in the centre of the upper surface of the disk ; and the ambulacral grooves 

 extend outwards between the bases of the orals, as the growing rays carry the fii'st 

 formed tentacles away from the water-vascular ring. 



In all the Pentacrinidte, and also in the Comatulaj, with the single exception of 

 Thawnatocrinus (PL LVI. fig. 5), the orals eventually undergo a process of resorption, 

 which commences in the hitter case before the young Comatula detaches itself from the 

 larval stem, so that no traces of the orals are to Ije found in the adult. Neither are 

 there any in the adult Bathycrinus aldrichiamis (PL VII. fig. 3), nor even in the young 

 Bathycrinus gracilis (PL Villa, fig. 1) ; though according to the observations of 

 Dauielssen and Koren they would seem to be present in Bathycrinus carjyenteri, liut in ;i 



1 Vergleichende Entwickehingsgeschichte der Comatula Mediterraiiea, Archir f. mikroal: Awil., V,d. xii. y. G21 



