MR. G. J. ROMANES AND PROFESSOR J. C. EWART 
830 
When, however, a coloured solution of gelatine is forced into the polian vesicle, the 
tentacles and their sinuses, the radial canals, ampullse, and pedicels are filled, as well 
as the circular canal and its sinuses. Examination of specimens injected with plaster 
of Paris and gelatine shows the circular canal to be a quarter of an inch in diameter, 
and to communicate freely both with the polian vesicle and with the lozenge-shaped 
sinuses springing from it. The specimens injected with a gelatine mass further show 
that each canal sinus opens into a ctecal tube, which runs forward internal to the 
sinuses of the tentacles as far as a wide circum-oral space (Plate 79, fig. 2, a). This 
space communicates by well-defined apertures (Plate 79, fig. 3, b), with that portion of 
the body cavity which lies between the sinuses and the oesophagus, and which is 
reached through the circular apertures between the sinuses of the circular canal 
(Plate 79, fig. 1,/). 
Each canal sinus has three other apertures in its walls. It opens by a small round 
aperture into a radial canal. The radial canal, together with the longitudinal muscular 
band, runs up between two of the sinuses of the tentacles to reach the inner surface of 
the body wall, and gives off lateral branches which project outwards as pedicels and 
inwards at each side of the longitudinal muscle as long-pointed ampullse (Plate 79, 
fig. 1, (j). The two other apertures are in the form of minute slits, one at each side of 
the orifice of the radial canal, which lead into the adjacent tentacle-sinuses. Each of 
these tentacle-sinuses measures three-quarters of an inch in length, and a little over 
a quarter of an inch in diameter. When the tentacle into which the sinus opens is 
protruded, there is no constriction between the sinus and the tentacle; but when the 
tentacle is retracted, there is a well-marked annular constriction (Plate 79, fig. 1, h) 
at the junction of the sinus with the tentacle, which may aid in preventing the fluid 
that is driven into the sinus during' retraction from again returning into and at once 
protruding the tentacle. If considerable pressure be applied to the polian vesicle 
when the tentacles are in a retracted state, this constriction disappears and the 
tentacles are distended, though not protruded. If the retractor muscles be divided 
while the polian vesicle is compressed, the tentacles become engorged and project 
forwards. On the other hand, when the pressure is removed from the vesicle and the 
divided retractors pulled backwards, the tentacles are approximated and, along with 
the group of sinuses and the anterior portion of the perisome, dragged backwards 
towards the centre of the body cavity. The eversion of the perisome and the pro¬ 
trusion of the tentacles are brought about chiefly by the shortening of the longitudinal 
muscular bands and the contraction of the polian vesicle ; but the circular fibres of the 
body wall also assist by contracting immediately behind the group of sinuses, so as to 
act on them by direct pressure, and also indirectly by forcing the perivisceral fluid 
against them. The fully distended position of the tentacles represented in figure 3 is 
only reached when the fluid of the body cavity has been forced into the circum-oral 
space (Plate 79, fig. 2, a). 
The amount of the body cavity fluid is constantly changing. At the entrance to the 
