360 CHARLES IL. BOULENGER. 
be described as claviform, but their shape varies according 
to the degree of elongation or contraction ; older specimens, 
also, bearing medusa-buds often have a greater relative 
diameter than younger ones. 
Distally the hydranth is provided with a prominent cylin- 
drical hypostome which is not constricted at its base. At 
the summit is the large mouth-opening surrounded by a 
narrow lip. At the proximal end the body tapers off 
gradually and passes imperceptibly into the narrow stem. 
Hach hydranth bears a small number of long, filiform 
tentacles arranged in a circle around the broadest part of 
the body below the hypostome ; they seem to develop singly. 
The number is very variable, and an individual may bear — 
from 0 to 8 tentacles; the majority, however, possess 4 or 5. 
When fully extended the tentacles are very slender, and 
may attain a length greater than twice that of the hydranths 
on which they are borne. 
The ectoderm of the hydranth consists chiefly of largish. 
epithelial cells, the basal portions of which are provided with 
conspicuous longitudinal muscle processes. Small inter- 
stitial cells are to be found between the larger cells, and 
nematocysts occur in abundance. ‘he ectoderm is deepest 
in the tentacle-bearing region of the body; in the hypostome 
it becomes much thinner, but again thickens around the 
mouth, where it forms the circular lip, crowded with large 
nematocysts. ‘he ectoderm of the tentacles is continuous 
with that of the body; the cells are, however, less regular in 
shape, and form ring-like thickenings on which the thread- 
cells are borne. 
The cells of the endoderm are by no means so uniform in 
structure as those of the outer layer. Roughly, three regions 
may be distinguished, corresponding well with those described 
by Hardy (1) in his paper on the histology of Myriothela 
phrygia. These regions are: 
(1) An oral region, situated in the hypostome ; here the 
endoderm presents four (rarely five) conspicuous longitudinal 
ridges formed by the greater elongation of certain of the 
