566 
PROEESSOR ALLMAN ON THE STRUCTURE 
cells, several in depth, the most internal presenting convex surfaces to the gastric cavity, 
but forming no villi-like projections. The ectoderm already consists of two zones besides 
the muscular lamella — a superficial zone composed of several layers of small round cells 
with clear granular contents, and a deeper zone of claviform tissue. The hyaline lamella 
with its muscular fibrillse lies everywhere between the claviform tissue and the endoderm. 
All these elements can be followed from the walls of the body into those of the arms. 
In these the endoderm, composed of small, round, clear cells, surrounds a wide axial cavity. 
When the arm has acquired its terminal capitulum, we find that the zone of claviform 
tissue, hitherto simply continued into the arm from the walls of the body, has become 
specially developed in the capitulum (Plate 56. fig. 13, c), and here envelops the 
endoderm in a nearly spherical cap, which takes exactly the place of the rod-like 
tissue in the permanent tentacles. The tissue composing this cap, moreover, is inter- 
mediate in its form between the ordinary clavate tissue and the rod-like tissue ; for its 
component elements do not form branching groups as in the clavate tissue of other 
parts, but consist of radiating, simple, greatly elongated clavate cells, very similar to 
those already described as forming the claviform tissue in the distal extremity of the 
clasper, and thus affording further evidence that the rod-like tissue is only a modified 
claviform tissue. 
The capitulum of the actinula arm further resembles that of the permanent tentacle 
in the presence of the pedunculated capsules. These differ, however, in some points 
from the corresponding organs of the permanent tentacles ; for they are not more than 
half their size, while the included cord is finer and longer, and is wound into closer and 
more numerous coils (Plate 56. fig. 14, a). Like the cord of the larger capsules, it 
continues after its emission to form a spiral, instead of straightening itself out in the 
field of the microscope like the filament of the true thread-cells. The spiral, however 
(fig. 14, b), is more open and more elongated than that formed by the cord ejected from 
the stalked capsules of the permanent tentacles. The styliform process of the external 
sac is long and slender. 
When the transitory arms of the Actinula have attained their full growth, the ecto- 
derm of their stem (fig. 13, a) no longer presents the two zones which were present in 
their younger stages. It is the superficial zone which appears now to be wanting, so 
that the clavate tissue comes to the surface. In thus becoming superficial the distal 
ends of the cells composing this tissue have become wider, and lie more closely on one 
another, and very often contain a large vacuole excavated in the midst of their granular 
contents. Their caudal prolongations, moreover, do not seem to run into one another 
to such an extent as to give rise to the botrylliform condition which characterizes this 
tissue in other parts of the animal. 
The endoderm of the arm (fig. 13, b) is formed externally by a tissue of large, clear, 
polygonal cells containing some minute granules, which are chiefly accumulated on the 
walls of the cells, while internally there is an irregular disconnected layer of small 
round cells filled with brown corpuscles. The increase of the endoderm in volume 
has nearly obliterated the axile canal of the arm. 
