REPOET ON THE HYDROIDA. 
XI 
tissue acquires increased development at tlie base of the tentacles, and thus forms 
a circular ridge which, pushing before it the mesosarc and gastral endoderm, projects 
into the gastral cavity of the hydranth. 
Though the axial tissue of the tentacles is thus separated by a layer of mesosarc 
from the endoderm which lines the gastral cavity, there is no reason why it should not 
be regarded as a special modification of the general endoderm of the body, and to speak 
of it, as Jickeli does, under the name of “ mesoderm,” as a third body layer appears to 
me to convey an erroneous view of its fundamental nature. 
This obliteration of the tentacular cavity, though almost universal in the hydranth, 
is far from being so in the marginal tentacles of the Medusa. Here, though in many 
cases (Trachomedusse and ISkircomedusse, Haeckel) the endoderm presents the rouleau- 
like condition, with obliteration of the lumen, in many others (Anthomedusse and Lepto- 
medusse, Haeckel) the axis continues pervious, and the endoderm forms a simple lining 
of the cavity with usually more homogeneous cell contents, but otherwise differing but 
little from that found in other parts of the body walls. 
Though the existence of muscular filaments in the ectoderm of the Hydroida has long- 
been known, it is comparatively lately that evidence has been adduced of their presence 
in connection with the endoderm. Weismann was the first to point out the existence 
of these endodermal fibrillse which he found to occur in Eudendrium J while the same 
have been found by Hamann 2 and by Jickeli 3 in other Hydroids. The fibrillse of which 
this musculature consists always run in a circular or transverse direction, thus 
contrasting with the fibrillse of the ectoderm which, at least in the trophosome, are always 
longitudinal. It is in the hypostome that they are most strongly developed, and here 
only do they occur, according to Hamann, in the Calyptoblastea, while in the Gymno- 
blastic genera they are found also in the walls of the gastric portion of the hydranth. 
They have not been detected in the ccenosarc. They would seem to be formed as 
outrunners from certain cells of the endoderm in a way similar to the formation of 
the longitudinal muscular fibrillse in the ectoderm, where, as we shall presently see, 
the fibrillse form outrunners from the most superficial cells of this layer. 
2. The Ectoderm. 
The ectoderm, like the endoderm, with which it is exactly co-extensive, and from 
which it is separated by the mesosarc, is also composed of nucleated cells. These 
cells are sometimes disposed in a single layer as in the ordinary condition of the 
endodermal cells, sometimes in several, while very frequently the cell-boundaries 
! Weismann, Ueber eigenthiimliche Organe, bei Eudendrium, Mittheil. aus der Zool. Stat. m Neapel, Bd. iii. 
2 Hamann, loc. cit. 
3 Carl F. Jickeli, Der Bau der Hydroidpolypen, Morpliol. Jahrb., Bd. viii. 
