CRINOIDEA 13 



The ambiilacral nervous system is only feebly developed and 

 not separated from the epidermis. On the other hand, the dorsal 

 or aboral nervous system is strongly developed. Within the 

 centro-dorsal it forms a conspicuous central organ, like a brain, 

 from which there radiates to each arm a branch, situated in a canal 

 in the centre of the calcareous joints, like the spinal canal of a 

 vertebral column. Also the nerves to the stalk and the cirri 

 arise from the central organ in the centro-dorsal. 



The genital organs are generally situated in the pinnules. 

 When the eggs are shed they remain in some forms {e.g. Antedon 

 bifida) in small clusters round the genital opening on the side of 

 the pinnule, and here pass through the first developmental stages ; 

 only when the larva has acquired its full shaj)e does it rupture the 

 egg membrane and become free. In others {e.g. Antedon petasus) 

 the eggs are free ; in this case the embryo is uniformly ciliated 

 when it ruptures the egg membrane and only later acquires the 

 ciliated rings characteristic of Crinoid larvae. The larva has no 

 mouth and so cannot take food, but subsists upon the yolk 

 contents of the egg. Within the larva the skeleton of the sea-lily 

 is formed, so orientated that the stalk lies in the anterior end, 

 the cal3^x in the posterior end of the larva. Thus the anterior 

 end of the larva becomes the posterior end (the stalk) of the sea- 

 lily, while the posterior end of the larva becomes the anterior 

 end of the sea-lil}", the mouth appearing there. After sum- 

 ming for some time, at most some few daj^s, the larva attaches 

 itself, with its anterior end, to some foreign object, an alga, a 

 hydroid, or the like. After some remarkable internal transforma- 

 tions the calyx oj)ens, the first tentacles protrude, and the young 

 sea-lily begins to feed. 



The skeleton of the calyx is at first formed as two Avhorls of 

 beautiful fenestrated calcareous plates, five in each whorl ;i 

 the}' are designated basals (lower whorl) arid orals (upper whorl) ; 

 the latter form five valves around the mouth. Soon a new 

 whorl of plates, the radials, appears, situated in the interstices 

 between the first two whorls, and thereafter, above the radials, 

 the first two ossicles of the arms, the brachials-^ ^^'y tbe second 

 brachial becomes the axillary, from which the arm bifurcates ; 

 ^ Generally a third whorl of very small plates develops below the basals, 

 alternating with these ; they are termed injrahasals. On account of their 

 small size and their being partly covered by the basals, they are not easily 

 observable, and their existence in the common British species Antedon 

 bifida has generally been denied. In reality they are well developed there. 

 As a rule the infrabasals are only three in number, two of them being some- 

 what larger, each representing two fused plates. 



