LTMNOCODIUM (CRASPEDACUSTES) SOWERBII. 361 
to separate the superficial ectodermal cells with their refringent 
nematocysts (thread-cells) from the immediately subjacent tissue, 
the cells of which have a yellowish-green tint, and are also some- 
what refringent. 
By careful focussing this difficulty is overcome, and it is then 
found that beneath the nettle-ring is a peculiar tissue, which 
forms the abumbral wall of the marginal canal (PI. XXX, 
The marginal canal itself is wide, and capable of distension on 
its abumbral side, where the wall is formed by cells of a very 
different character from that of those which underlie the nettle - 
ring. In fig. 5 the superior wall of the marginal canal {MCa) is 
seen in section, showing the peculiar soft clear (ciliated) cells 
containing yellowish-green granules, by which it is here lined 
internally. The modified cartilaginoid cells of the abumbral 
wall are limited inferiorly {i, e. taking the mouth as indicating 
the inferior surface of the organism) by a strongly-marked row 
of quadrate cells, which form a slight ridge, and are especially 
observable by their complete and uniform pigmentation (PI. 
XXX, fig. 3 VMCy fig. 4 VMCy fig. 5 MCp ) — yellowish green or 
greenish brown. These cells form a dark line, which can alw^ays 
be recognised along the insertion of the velum, and accordingly 
I distinguish them as velo-marginal endoderm. 
The relations of these parts are exhibited, in a diagrammatic 
form, in PI. XXX, fig. 3. A comparison of this figure, with 
the surface view and the optical section, will, I hope, serve to 
make the structure comprehensible. 
The true character of the tissue formed by the endodermal 
cells of the abumbral wall of the marginal canal is best appre- 
ciated by tracing the gradual transition of the tentacle root into 
this tissue. 
The axis of the tentacle is formed by entoplastic cartilage 
cells, that is to say, cells the protoplasm of which undergoes 
a cartilaginoid metamorphosis within the cell area, as in 
the notochord of Vertebrata. 
The tissue very closely resembles that of the notochord of a 
young Lam])rey. It differs in the smaller density of the cartila- 
ginous deposit, as also in the greater abundance of the cells, 
from that of typical Trachyline. Medusae, and resembles that of 
tlic solid tentacles of many hydroid polyps. At the same time 
there can be no question that these tentacles more closely 
resemble those of Trachomedusm than they do those of Le.])to- 
medusse, since iji all Leptomedusse the tentacles arc hollow, 
whilst in Trachomcdusae the tentacles are very usually solid, as 
they arc here. 
In that part of the tentacle axis which is plunged in the disc, 
