

CEINOIDEA 



rd 



dividual >e-mcnt> together. Tin- union i> further M ren-t hem-d l.y siituiv>. 

 the appos.-d faces >f tin- joint- lii-in^ tor tin- most |iart fiat, and cither radially 

 -triated, 01- diversified in \arioiis ways l.y ]>roinine!ices and ; he ^roo\ > 



In-ill-' likewise occupied l.y conneeti\e tissue. Smile! ime> tin- :I|J|M.-.-. : 

 are -niooth and rigidly united with one another ( /'////// //>-, etcj ; l.nt in 

 Other cases an articulation is >emred l.y means of an elevated usually oblique 

 fulcral i-id^f (/////://////.<, /Ai// ////'/"//////>). The proximal segments ot the 

 eoliinin occasionally exhibit sini|.le vertical clefts whi-h indicate an original 

 |iiiiH|iii]iartite composition. These divisions always occur alternately with 

 those of the l.asals in monocyclic, and with those of the infrabasals in dicydie 

 forms. 



(hdn^uij. Although \\e are acquaint ed as yet with the life -history of 

 I. ut the single genus .////MA///, the phenomena of its development are of such 

 H-niticam e as to shed most valual. le light upon many con- 

 ditions ol.served in fossil Crinoids. Following impregnation, 

 the ova perform the early stages of development within tin: 

 ovarian glands of the j.innules. Immediately after escaping 

 from the \itelline membrane the embryo is girded with four 

 tran>\er>e ciliated Lands, 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 

 >eiie> being plaeed symmetrically above those of the posterior. 

 K\ tend in- from the centre of the lower row downwards and 

 backwards, a series of delicate calcareous rings may now be 

 observed, at tlie 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 

 >een pa-Hii- through tly rin-s of the rudimentary stalk. The 

 hitherto free-swimming larva now becomes fixed, the ciliated 

 bands disappear, the outer surface becomes invested with a 

 thick sari-ode layer, and the mouth appears as a lar^e patent 

 opening "eiipyin:;- the centre of the cup. The five plates 

 surroundin.u the mouth are termed the orals, and the five con- * al , *,.:,]. 

 stitutin^ the proximal series the basals, which rest directly dowd 

 upon the tupnio>t segment (centrodorsal) of the stalk. In 

 the five diamond-shaped spaces which are left between the divisions of the 

 urals and ha>al> delicate net like plates ( ra dials) are next formed, which increase 

 rapidly in si/e 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 





s] 



w.wiii.- 



