526 
PROFESSOR W. THOMSON ON THE EMBRYOGENY OE ANTEDON 
A wide vascular ring surrounds the mouth, occupying nearly the whole of the space 
between the lip and the base of the oral lobes. This ring seems to be simply hollowed 
out in the uniform sarcode. Its walls are not contractile, it maintains a constant 
diameter of about 0'08 millim. It is filled with a transparent liquid, which passes like- 
wise into all its tubular appendages ; and as granules move rapidly in this fluid, the 
walls of the ring would seem to be ciliated, though hitherto no cilia have been detected, 
even in sections and under high powers. The upper and outer margin of the ring gives 
origin to two classes of tubular tentacles. In a very few cases in which I had an oppor- 
tunity of looking into the cup immediately after its expansion, the total number of these 
appendages has been fifteen, five extensile, and ten non-extensile. I have never seen 
fewer ; and I feel convinced that these, with the vascular ring from which they spring, 
are developed towards the close of the pseudembryonic stage and within the closed cup ; 
they are protruded so immediately after its first expansion. 
Radially, the ring gives off five highly mobile, irritable, and extensile tubular tenta- 
cles, one opposite each of the intervals between the oral lobes. The cavity of these 
tentacles is continuous throughout, and immediately continuous with the cavity of the 
oral ring. Their wall seems to consist of a simple contractile sarcode-layer, studded 
with oval yellowish endoplasts. There is no definite differentiation of a contractile 
fibrous tissue. Under a high power, however, the sarcode appears to have a longitu- 
dinal arrangement ; this may possibly be due to motion among the particles producing a 
play of light. The walls of these tentacles are produced into numerous delicate tubular 
processes (Plate XXVI. fig. 3 e), their cavities continuous with those of the tentacles. 
These processes are arranged in three or four irregular longitudinal rows. They are 
extensile, their walls when extended are extremely delicate, transparent, and apparently 
structureless. When contracted two or three delicate ring-like rugae appear on the 
walls of each (Plate XXV. fig. 3). Each process is terminated by a minute three- 
lobed slightly granular head. At the base of each of these processes there is a delicate 
crescentic leaf-like fold, slightly granular, and most distinctly marked when the tentacle 
is retracted. When one of the extensile tentacles is wholly or partially retracted, it is 
thrown into obscure transverse wrinkles, which give it at first sight the appearance of 
being divided by a series of dissepiments. When the tentacle is fully extended these 
folds totally disappear. At the base of each of these five “ azygous tentacles ” there is a 
conical thickening and enlargement of the sarcode-tissue, contracting outwards towards 
the tentacle which is continuous with its apex, and whose cavity passes through it to 
unite at its base with the oral vascular ring. This conical projection is the commence- 
ment of the young arm. The azygous tentacle terminates it, and leads it out, as it were, 
up to the point of bifurcation. The tentacle remains persistent for some time in 
the angle between the two first brachial joints (Plate XXVII. figs. 1 & 3), and finally 
becomes absorbed and disappears. These five azygous tentacles are the first of a 
system of “ extensile tentacles” which are subsequently developed in very extended 
series as appendages of the radial and brachial tentacular canals. In almost all cases, 
