MONOGRAPH OF THE EXISTING CRINOIDS. 501 



body than the two ventral basals. These three infrabasals are at first equal in 

 size, but after a while two of them bejiin to grow more rapidly than the third, 

 eventually attaining to about double its size. 



At a later stage the inner border of each becomes smooth and concave, and 

 they then range themselves in a circle around the chambered organ, just above 

 the topmost columnal. The arrangement of the plates is still the same as in the 

 earlier stage. Somewhat later they fuse with one another and with the topmost 

 columnal so as to form one large plate. Though the sutures of the infrabasals 

 still persist, the plates themselves have grown out into five angles which are 

 radial in position, fitting in between the bases of the basal plates. While the 

 dorsal infrabasal produces only one angle, each of the other two grows out into 

 two angles, this fact strongly suggesting that each of these larger infrabasals 

 represent a pair of coalesced plates, and that the primitive arrangement would 

 be the possession of five radial infrabasals. At a slightly later stage the sutures 

 disappear, though the groove separating the infrabasal series from the topmost 

 columnal persists for some time. 



Tims in reality the topmost columnal only forms the lower half of the centro- 

 dorsal, the upper half being formed by the infrabasals. 



Bury records that the larvae showed a singular want of discrimination in their 

 choice of positions for fixation; besides numbers which perished from attaching 

 themselves to objects too small to support them, some attached themselves to the 

 pinnules of the parent, where they never managed to adhere very long, and others 

 to their own cast-off vitelline membranes, which were soon eaten up by bacteria 

 and infusoria. 



The ease with which the vitelline membrane can be broken is subject to 

 considerable variation. Some whole broods are set free with ease on the fourth 

 day; but in most broods the majority are not set free until the seventh day, and 

 a considerable number are unable to free themselves even on the ninth or tenth 

 days, long after the majority of the larvae have become fixed. Such prisoners, 

 when at length liberated, are seldom able to fix themselves, and if they do they 

 usually exhibit considerable abnormalities of form and either grow into monstrous 

 shapes or remain perfectly spherical. Some of the last Bury liberated as late as 

 the eleventh day, and though they had lost the two ventral depressions, they 

 still retained some of their cilia in active motion. 



Bury writes that the reason of so much variation in the consistency of the 

 vitelline membrane, leading so often to considerable loss of life, is not easy to see, 

 but it may possibly be connected with the varying conditions to which these animals 

 are exposed. Thus if the membrane were always of the same strength a sudden 

 storm might set all the larvae free at too early a stage of development, and so 

 deprive them of the protection afforded by the pinnules of the parent, while if the 

 membrane were strong enough to resist a storm, a continued calm might leave a 

 greater number unable ever to free themselves than at present occurs. 



The first changes which occur after fixation are principally histological, and 

 it is not for some hours that the general internal anatomy alters much. The 



