Ii6 Papers from the Marine Biological Laboratory at Tortugas. 



for no especial comment. In the young vertebra of angulata (pi. 2, fig. 7), 

 besides the excessive length, we note the very low proximal (adoral) and the 

 very high distal (aboral) ends, the slight development of the alae, the well- 

 developed aboral and median hypapophyses, the conspicuous protapophysis, 

 and a pair of hollowed projections above the zygosphene, which we may 

 call the zygocceles. In the later development of the vertebra the length 

 only increases about one-half, while the height anteriorly increases 3.5 

 times, and posteriorly more than 6 times. The adult vertebra is about as 

 high as it is long, while the width of the alae exceeds the total length. Seen 

 from above (pi. 2, fig. 9), the most striking feature is the very narrow 

 protapophysis, the immense zygantrum, the very small and insignificant 

 epanapophysis, the reduced epapophyses (particularly the median one), 

 the broad parapophyses and hypapophyses. Seen from the side (pi. 2, 

 fig. 8) the great height of the protapophysis, the bulging zygocceles, the 

 long zygosphene, and the prominent hypapophyses, all attract attention. 

 Looking at the vertebra from below (pi. 2, fig. 1 1), the breadth of the taphrus 

 and zygosphene are the most notable features. Looking at the aboral end 

 (pi. 2, fig. 10) we are again struck by the high, narrow protapophysis, the 

 conspicuous zygosphene and the zygocceles, while the broad alae furnish 

 a fitting background. At the adoral end (pi. 2, fig. 12) the reduced area of 

 the alee caused by the huge zygantrum, the small zygotreme, within which 

 is a small knob that may be called the zygophore, and the large adoral zyga- 

 pophyses are the principal features. We may then say that the adult 

 vertebra is short and high, as in Ophiactis, but, in contrast to that genus, 

 the zygantrum is large, the protapophysis is narrow, high, and projecting, 

 the epanapophysis is insignificant, and the adoral hypapophysis is large. 

 The joint formed by two vertebrae is more complicated than that in 

 Ophiactis. The protapophysis of each vertebra fits into the zygantrum of 

 the next distal segment and a compact mass of muscles, attached at their 

 outer ends to the sides of the cavity, hold the protapophysis in position; 

 the inner ends of these muscles are attached on either side of the prota- 

 pophysis. The zygocceles serve as sockets for the zygapophyses, while the 

 epanapophysis of the distal vertebra rests lightly on them, the zygosphene 

 meanwhile fitting into the zygotreme and resting against the zygophore. 

 The aboral hypapophyses fit outside and beneath the adoral pair of the 

 distal segment. This articulation is, therefore, considerably more complex 

 than the one described on p. 102 and yet it may permit just as much freedom 

 of movement. The lateral flexures of the arm could certainly be made 

 more vigorously and a curved position held more rigidly than in forms with 

 less-developed zygantra and protapophyses. Possibly the reduced epana- 

 pophysis permits a greater freedom in vertical movements, but it may be 

 that the larger adoral hypapophyses counteract this tendency. 



Although the development of the vertebra is along the same lines as in 

 Ophiactis, there is a constantly increasing difi'erence between that genus and 

 Ophiothrix. After the two halves of the vertebra have come to lie close 



