ON THE LOCOMOTOR SYSTEM OF ECHINODERMATA. 
8G9 
not co-operate to move the animal away from the source of irritation, as is so invari¬ 
ably the case witli unmutilated specimens. Removal of the nerve-ring lias entirely 
destroyed the general co-ordination of the spines. 
GENERAL SUMMARY. 
I. Morphology. 
In Holothuria the polian vesicle opens freely into a wide circular canal a short 
distance from the termination of the stone canal. From this circular canal five 
lozenge-shaped sinuses project forwards, and from each of these two large oval sinuses 
run forwards parallel with each other—the ten oval sinuses becoming continuous with 
the hollow stems of the tentacles. Injection of the polian vesicle shows that it forms 
one continuous tube system with the circular canal and its sinuses, oval sinuses and 
tentacles, pedicels and ampullae. Unless the pressure is kept up for a considerable 
time there is no penetration of the injected fluid into the stone canal, and either the 
ring, vesicle, or sinuses, give way before the fluid reaches the madreporic plate. 
Specimens injected with a gelatine mass show that each canal sinus opens into a caecal 
tube, which runs forwards internal to the sinuses of the tentacles as far as a wide 
circum-oral space. This space communicates by well-defined apertures 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. Each canal sinus has three other apertures in its walls. It opens by a small 
round aperture into a radial canal, and the two other apertures occur as minute sliis, 
one at each side of the orifice of the radial canal, leading into the adjacent tentacle 
sinuses. 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 constriction at the junction of the sinus with the tentacle. 
The eversion of the perisome and the protrusion of the tentacles are effected chiefly 
by the shortening of the polian vesicle and the constriction of the longitudinal 
muscular bands, which run from the inner surface of the body-wall between each two 
adjacent tentacle-sinuses ; but the circular fibres of the body-wall also assist in the 
process by contracting immediately behind the group of sinuses, so as to act on them 
by direct pressure, and also indirectly by forcing the body fluid against them. 
The amount of the body-cavity fluid is constantly changing. At the entrance to 
cloacal chamber there is a circular valve which is constantly dilating and contracting, 
except when the aboral end of the animal is forcibly retracted. When open, this valve 
allows water to pass into the respiratory tree; when it begins to retract, water 
escapes from the cloaca. This alternate opening and closing takes place with perfect 
rhythm, at a rate of about six revolutions per minute. At the end of every seventh 
or eighth revolution a large stream of clear water is ejected, which sometimes contains 
