REPORT ON THE CRINOIDEA. 71 
diately surrounding the peristome and covering it more or less completely (PI. III. 
no 2> Pl Vege GOsailoVls figs: 3, 4; PO Xo figs. 73203) Pl LVL 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 tube, 
and contains the fifteen first formed tentacles which are borne on the water-vascular 
ring, At a certain period of development the five valves of this oral pyramid gradually 
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, «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 continually 
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 first 
formed tentacles away from the water-vascular ring. 
In all the Pentacrinide, and also in the Comatule, with the single exception of 
Thaumatocrinus (Pl. LVI. fig. 5), the orals eventually undergo a process of resorption, 
which commences in the latter case before the young Comatula detaches itself from the 
larval stem, so that no traces of the orals are to be found in the adult. Neither are 
there any in the adult Bathycrinus aldrichianus (Pl. VII. fig. 3), nor even in the young 
Bathycrinus gracilis (Pl. VIIa. fig. 1); though according to the observations of 
Danielssen and Koren they would seem to be present in Bathycrinus carpenteri, but in a 
1 Vergleichende Entwickelungsgeschichte der Comatula Mediterranea, Archiv f. mikrosk. Anat., Bd. xii. p. 621. 
