MINUTE ANATOMY OF THE BRACHIATE ECHINODERMS, 179 
plates (Bu’.); while the abradial margin of the cleft is formed 
by a continuation of the general scaly covering of the ven- 
tral perisome (Bu’’.), The walls of the sac sometimes contain 
small calcareous plates. Into each bursa open the isolated 
Fig. 1.—Diagrammatic vertical section across a radius of an Ophioglypha 
near the edge of the disc. Kd. dorsal; Kv. ventral body-wall; MZ. 
radial diverticulum of the stomach; 4. portion of the arm included in 
the dise; B. bursa; Bw’. the adradial edge of the bursal cleft, with the 
bursal (genital) plate; Bw’, its abradial edge, with the scales of the 
ventral perisome ; G. genital tubes. The arrows point to the bursal 
clefts. (Copied from Ludwig.) 
genital tubes (G.), some of which occupy the space between 
it and theray, while others are situated on its abradial side, 
extending nearly up to its dorsal surface. Each tube is 
surrounded by a blood space, which is derived either directly 
from the aboral ring, as in the case of the abradial tubes, or 
from the genital vessels (adradial tubes). The genital pro- 
ducts are not set free into the body-cavity, as was formerly 
supposed, but merely into the burse from which they 
reach the exterior. It is possible that the burs are also 
respiratory organs, serving the same purpose as the external 
respiratory ceca of the Asterids; while in some viviparous 
species they serve as marsupial cavities.! 
Among recent Hchinoderms there is no structure at all 
comparable to these burse of the Ophiurids, but Ludwig 
points out their resemblance to the hydrospires of the Blas- 
toids, and especially those of Orophocrinus (Codonites). In 
this genus the hydrospires have no special spiracular 
openings at the apex as in Pentremites. There are also 
1 Studer has recently pointed out (‘ Zool. Anzeiger,’ No. 67, p. 526) that 
he described these bursz as marsupial pouches as long ago as 1876 (‘ An- 
tarkt. Kchinodermen. Monatsber. d. Berlin. Akad.,’ 1876). In Ophiomyaa 
vivipara every pouch contains two or three fully-developed young starfishes, 
each enclosed in a thin membrane like a chorion. Ophiacantha vivipara 
has fourteen clefts aud the same number of pouches, in each of which 
there may be three young ones. 
