MOEPHOLOGICAL PAET. 99 



thus considerably larger than the plates occii|)ying its outer margins, which 

 are also exposed in that specimen. 



We thus find persistent among Palgeozoic 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 Hyommis (Plate III, 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 Palgeozoic 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 Palgeo- 

 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 Actmometra, 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 subcentral 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. ZooL, Vol. XVII, Plate 6, Pigs. 1 and 2. 



