MONOGRAPH OF THE EXISTING CEINOIDS. 447 



LAST PERIOD OF EMBKYONAL DEVELOPMENT; FORMATION OF CALCABEOUS SKELETON (84 TO 120 HOOKS). 



During the fifth clay the embryo escapes from the egg membrane and becomes 

 a free-swimming larva. It is now elongated in the direction of the major axis 

 and has reached a length of about 0.40 mm. As a result of the advance of the 

 vestibular furrow the general form has become beanlike. 



Up to the time of the appearance of the calcareous skeleton the inner organs 

 remain essentially as just described, but the walls are composed of smaller cells, 

 and the lumina are more extensive. 



Owing to the exceptional opaqueness of the living object it is necessary to study 

 the skeleton in preserved material. 



In embryos of 100 hours there are already present five orals, five basals, and 

 from three to five infrabasals in the calyx, and about 11 calcareous elements in 

 the column. 



It appears that in the embryos of animals kept in aquaria for some time 

 before the extrusion of the eggs the calcareous structures are less extensively 

 developed, smaller and thinner, and at the same time very much more variable, 

 than is the case in embryos from mothers living freely in the sea. 



The calcareous plates are often quite irregularly developed in animals of 

 exactly the same age, and the individual plates of the same series in the same 

 animal often show considerable differences. 



Each of the five basals, like each of the five orals, is approximately equidistant 

 from those on either side, except that in each series the distance between the two 

 outermost is greater than the distance between any two of the others. Thus 

 it is that the basals and orals have usually been described as lying in horseshoe- 

 shaped bands, open ventrally, surrounding the major axis. If it is borne in mind 

 that the planes of the two horseshoes are strongly oblique, so that the open sides 

 lie much more anteriorly than the closed ends, and that the left arms of the 

 horseshoes in each case reach farther anteriorly than the right, the relative positions 

 of the plates in these two series are fairly well indicated. 



At this stage the individual basals and orals do not lie exactly behind each 

 other, for the basals are slightly more displaced dorsally than the orals. Fre- 

 quently this displacement is such as to bring the first basal almost or quite in line 

 with the second oral. 



The left basal (JJ lies farther anteriorly than the right (Z> 5 ) and the left oral 

 (oj in the same way lies farther forward than the right (<? 5 ). Usually, the right 

 basal lies farther anteriorly than the left oral. 



The plates in these two series lying farthest posteriorly are those in the 

 median plane dorsally, the third oral and the third basal. 



While the first and fifth basals lie in the region delimited by the third ciliated 

 ring, usually just proximal or just distal to it, the third basal lies under the fourth 

 ring. In the same way while the first and fifth orals lie under the fourth ring, 

 the third lies under the fifth. 



