MORPHOLOGICAL PART. 99 
thus considerably larger than the plates occupying its outer margins, which 
are also exposed in that specimen, 
We thus find persistent among Palzozoie Crinoids all the phases through 
which the orals pass in their individual growth in recent forms, from the 
early Pentacrinoid larva of Antedon to the adult Hyocrinus (Plate IIT, Fig. 10) 
in which they are very large, and Calamocrinus * in which they are extremely 
small; and we find the plates in process of resorption and entirely removed 
from the system. 
We further find that the orals in all Crinoids, recent and fossil, when 
represented, occupy the centre of the disk, immediately surrounding the 
mouth or covering it, and that the orals of the earlier forms differ from those 
of the recent only in their asymmetrical arrangement, caused by the greater 
rigidity and more extensive development of the anal structures. 
B. Mouth and Ambulacra. 
The presence of a single aperture in the disk of Paleozoic Crinoids 
induced the earlier writers to suppose that this opening, although interradi- 
ally disposed, served both as mouth and vent. Later observations, and 
a better knowledge of the general structure of recent Crinoids, their mode 
of feeding and the nature of their food, have shown conclusively that this 
opening is not the mouth, but the anus, and that the mouth in most Palxo- 
zoic forms was subtegminal. 
The mouth of all Crinoids is directed upwards, being placed in the centre 
of radiation, but does not in all of them occupy the centre of figure. It is 
very frequently subcentral, and may be altogether excentric. The latter is 
the case in the asymmetrical genus Actinometra, and to some extent in all 
Fistulata, in which the posterior side of the disk is extended into a large 
tubular or sac-like prolongation. It is subeentral in most of the Camerata, 
and central in all known recent forms, Actinometra excepted. 
The ambulacra occupy the grooves along the ventral side of the arms, 
and extend from the tips of the pinnules to the mouth. Their proximal 
ends are either exposed upon the disk, or covered wholly or in part by 
plates of the tegmen. Entering the mouth there are five main trunks, 
which ramify so as to give a branch to every arm and pinnule. The upper 
face of the ambulacra is occupied by the food grooves, which are roofed over 
* A. Agassiz, Memoirs Mus. Comp. Zodl., Vol. XVII, Plate 6, Figs. 1 and 2. 
