CLASS I CRINOIDEA 131 
dividual segments together. The union is further strengthened by sutures, 
the apposed faces of the joints being for the most part flat, and either radially 
striated, or diversified in various ways by prominences and grooves, the grooves 
being likewise occupied by connective tissue. Sometimes the apposed faces 
are smooth and rigidly united with one another (Hucrinus, ete.) ; but in 
other cases an articulation is secured by means of an elevated usually oblique 
fuleral ridge (Lhizocrinus, Bourguetocrinus). The proximal segments of the 
column occasionally exhibit simple vertical clefts which indicate an original 
quinquipartite composition. These divisions always occur alternately with 
those of the basals in monocyclic, and with those of the infrabasals in dicyclic 
forms. 
Ontogeny. — Although we are acquainted as yet with the life-history of 
but the single genus Antedon, the phenomena of its development are of such 
significance as to shed most valuable light upon many con- 
ditions observed in fossil Crinoids. Following impregnation, 
the ova perform the early stages of development within the 
ovarian glands of the pinnules. lmmediately after escaping 
from the vitelline membrane the embryo is girded with four 
transverse ciliated bands, possesses a laterally situated mouth, 
and bears considerable resemblance to certain Annelid em- 
bryoes. Delicate plates of calcareous network (which, when 
definitely developed, are ten in number) are next formed 
within the wider anterior portion of the gastrula larva ; these 
are arranged in two transverse rings, the plates of the anterior 
series being placed symmetrically above those of the posterior. 
Extending from the centre of the lower row downwards and 
backwards, a series of delicate calcareous rings may now be 
observed, at the base of which a large cribriform plate is 
rapidly developed. The two rows of plates gradually become 
enlarged, and the outline of the rudimentary calyx is more 
clearly defined, while sheaves of parallel calcareous rods are 
seen passing through the rings of the rudimentary stalk. The 
hitherto free-swimming larva now becomes fixed, the ciliated 
bands disappear, the outer surface becomes invested with a 
thick sarcode layer, and the mouth appears as a large patent 
opening occupying the centre of the cup. The five plates jomne ihe Ne 
surrounding the mouth are termed the orals, and the five con- 5%163,.7 Radials : 
stituting the proximal series the basals, which rest directly dorsal (after Wyville 
upon the topmost segment (centrodorsal) of the stalk. In wast 
the five diamond-shaped spaces which are left between the divisions of the 
orals and basals delicate net-like plates (radials) are next formed, which increase 
rapidly in size and intrude upon the orals. At the same time a sixth plate 
(anal) makes its appearance in the zone of radials, but is gradually forced 
upward with the orals into the ventral disk. Rows of elongate cylindrical 
segments (brachials), which grow very rapidly, are next given off by the 
radials, when the larva is said to have reached the “ Pentacrinus stage.” 
Simultaneously with the development of the arms and column, a resorption of 
the anal and orals occurs, which plates are obliterated on the completion of the 
dermal skeleton. The basals also become obsolete, owing to the encroachment 
of the centrodorsal, and are indicated in the adult form by a mere rudiment in 


