ON M@RISTA LYONSI. 361 
constituent cells, which are of two kinds:—(a) Elongated 
conical cells with finely granular protoplasm and a small 
nucleus situated in the tapering basal part; (b) narrow 
palisade-like cells, between the distal ends of which the 
former are wedged in; the protoplasm is clear and scarcely 
stains ; their bases bear transverse muscle processes em- 
bedded in the mesoglea. 
(2) The second region is that from which the tentacles 
arise. ‘The palisade cells are replaced by strongly vacuolated 
cells likewise arranged in longitudinal ridges ; the latter are, 
however, less conspicuous than in the oral region, and are, 
moreover, far more numerous; aS many as twenty may be 
counted in a transverse section of alarge hydranth. Between 
the vacuolated cells are situated numerous very characteristic 
club-shaped gland-cells with large nuclei and coarsely- 
granular protoplasm. : 
The endoderm of the tentacles is continuous with that of 
the gastric cavity ; in a transverse section of these organs we 
see five or six large clear cells surrounding a small but dis- 
tinct tentacle cavity (fig. 8, Tent. Cav.). In this respect 
Meerisia resembles Hydra and differs from all known 
colonial hydroids, for in these the long axis of the tentacle is 
occupied by a solid core of large vacuolated cells of so-called 
“ notochordal” structure. 
(8) In the third region, at the narrow base of the hydranth, 
we find cells very similar to those described as occurring in 
the tentacle zone. The ridges have, however, become quite 
inconspicuous and gland-cells are very rare. 
As before mentioned, nematocysts occur scattered about in 
the whole of the ectoderm of the hydranths. Four kinds can 
be distinguished, similar in most respects to the forms which 
have been described in Hydra (2). They are: 
(1) A large, oval, barbed nematocyst (fig. 9, a, b) found 
in great abundance on the body, hypostome and tentacles ; 
most numerous in the ectodermal lip which surrounds the 
mouth opening. When everted the thread is seen to possess 
a thickened basal portion on which are developed a proximal 
