364 
PROFESSOR E. RAY LANKESTER. 
averaging of an incli in diameter, enclosed in an 
elongated tubular capsule. The refringent body is situated at 
the line of junction of the velum with the marginal ring, within 
the slightly enlarged termination of the tubular capsule. 
The tubular capsule is continued thence radially to the 
extreme margin of the velum, whence I have termed these 
capsules ‘Welar canals^'’ (PI. XXX, fig. 1). 
The refringent body belongs essentially to the abumbral side 
of the velum— that which is adjacent to the tentacles — as will be 
described below, whilst the capsule is entirely formed by the 
ectodermal cells of the abumbral surface of the velum. 
The refringent bodies and their capsules form interruptions in 
a cellular band of thickened ectodermal tissue, which runs along 
the line of junction of the velum and disc peripherally to the 
strongly-coloured velo-marginal cells of the endoderm of the 
marginal ring. 
The appearance of this ring of transparent, colourless cells, 
and the relation of the marginal bodies and their capsules to it, 
is seen in PL XXX, fig. 5, ca. The transparent ring (^r) is 
undoubtedly the representative of the abumbral nerve-ring of 
other Hydromedusse ; but I have not yet made that special his- 
tological study of its elements which would enable me to say 
how much of the ring is to be regarded as nervous tissue, and 
how much is unspecialised ectoderm. ^ 
The tubular capsules of the marginal bodies are often to be 
found in an incomplete state of development, and it is then 
possible to observe the process by which they increase in length, 
so as to reach finally the edge of the velum. The tubular cap- 
sules occupy a position between the abumbral and adumbral 
ectoderm layers which constitute the velum, and they actually 
stand out on the abumbral surface of the velum as ridges raising 
up the abumbral ectoderm of the velum, and separating it en- 
tirely from the adumbral ectoderm in the form of a delicate 
membrane. This is well seen when the velum is folded on 
itself, as not unfrequently happens when a specimen of the 
Medusa is placed beneath a cover glass, as shown in PI. XXX, 
fig. 2. 
If the tubular capsules be now carefully studied it is not 
difficult to make out that they increase in length, not by the 
simple growth of the already existing capsule, but by the fusion 
with the capsule of vacuolated cells belonging to the abumbral 
layer of the velum (fig. 2 vac'). 
The vacuolated cells of the abumbral ectoderm layer of the 
velum may be seen in various regions of the velum, ready, as it 
were, to fuse with the capsules, should their growth favour such 
an occurrence. The tendency to fuse on the part of the cap- 
