Growth-changes in Brittle Stars. loi 



Taking the fully formed vertebra of Ophiactis as a type (pi. i, figs. 

 14 to 18), we may distinguish the following parts, when the vertebra is 

 seen from above (fig. 14) : the alee form the bulk of the adoral end, their 

 upper margins forming the upper alar ridges; these alar ridges are widely 

 expanded at their inner end, but do not meet in the mid -line; the space 

 between is of little importance in Ophiactis, but is very conspicuous in some 

 genera ; it may be called the zygantrum; the expanded inner end of each alar 

 ridge is more or less clearly divided into three parts which may be called the 

 epapophyses; on each side there is an aboral, median, and adoral epapophysis 

 and in Ophiactis they are all distinguishable, although the aboral is the most 

 conspicuous. Directly in front of the aboral epapophysis is a broad, nearly 

 flat platform, the protapophysis, the aboral corners of which are higher 

 than the median portion. Projecting from beneath this median portion 

 may be seen the zygosphene, a shining, glassy knob which still shows evidence 

 of its paired origin. At either side, but still lower, are the aboral hypapo- 

 physes; back of these and about half-way between them and the alse are the 

 parapophyses, not very conspicuous or important ridges closely associated 

 with the median hypapophyses ; back of and below the zygantrum is a 

 conspicuous median, unpaired process, the epanapophysis, which is particu- 

 larly noticeable in the vertebrae within the disk (pi. i, fig. 19) ; on either side 

 of the epanapophysis, but in a much lower plane, lies a zygapophysis. 



These are all the parts which are visible from above, but if we look at 

 the adoral surface of the vertebra, (pi. i, fig. 17) we shall see below the 

 zygapophyses, the adoral pair of hypapophyses, while between the zygapo- 

 physes is the important space, the zygotreme. Looking at the lower surface 

 of the vertebra (pi. i, fig. 16) a deep furrow, the taphrus, is seen running from 

 between the zygapophyses to the zygosphene ; it is bounded on either side 

 by the three hypapophyses and the thin, vertical wall which connects their 

 bases; the taphrus contains the radial water- vessel and nerve; the lower 

 alar ridges form the ventral margin of the alae. 



Making use of these terms, then, we may describe the vertebra of 

 Ophiactis as short and high, with a small zygantrum, a broad, low prota- 

 pophysis, a well-marked zygosphene, a large epanopophysis, well-developed 

 zygapophyses, and small, adoral hypapophyses. The basal segments of 

 the arm are so crowded and compressed that they do not fairly show the 

 appearance of a typical vertebra, but from the tenth to the fifteenth seg- 

 ment the vertebra show clearly their characteristic features. Beyond the 

 twentieth segment youthful characters become evident and these become 

 more and more marked as one approaches the tip of the arm. This is well 

 brought out by comparing a basal joint of a young arm with a corresponding 

 joint of an adult arm. For example, the resemblance is very noticeable 

 between the tenth vertebra (counting from the base of the arm outward) 

 of an arm having 18 joints (pi. i, fig. 12) and the thirtieth vertebra of an arm 

 with more than 60 joints (pi. i, fig. 11), but the difference between this 

 young tenth vertebra and the tenth vertebra of the adult arm (pi. i, fig. 14) 



