ON THE LOCOMOTOR SYSTEM OF ECHINODERMATA. 
831 
cloacal chamber a circular valve is seen alternately dilating and contracting, except 
when the aboral end of the Holothurian is forcibly retracted. When this valve dilates 
it lies in close contact with the walls of the cloaca (Plate 79, fig. 4) and allows water 
to enter for the respiratory tree. It remains open for a few seconds and then contracts 
so as to project beyond the aperture (Plate 79, fig. 5); when it begins to retract and 
dilate, water escapes from the cloaca. This alternate opening and closing takes place 
rhythmically, at a rate usually of six revolutions per minute. At the end of every 
seventh or eighth revolution the valve projects further than usual, and, while it is 
slowly dilating, a large stream of clear water is ejected. The escape of this stream 
occupies from 15 to 20 seconds. Occasionally along with the stream a quantity of 
sand and the remains of the food particles are carried out from the cloaca. When 
the tentacles are being protruded the rhythmic action of the valve goes on as before, 
but more water is taken in than escapes from the cloaca; on the other hand,retraction 
of the tentacles is preceded by an escape of a large stream of water, and while retrac¬ 
tion is proceeding more water escapes than when the Holothurian is at rest with its 
tentacles projecting. 
2. Echinus. —In Echinus ( E . sphcera and Jiviclus ) two tubes spring from the under 
surface of the madreporic plate. The one (Plate 80, fig. 10, a) is dilated at its origin so 
as to include the greater portion of the plate (Plate 80, fig. 10, h), and ends in the so- 
called “heart” (Plate 80, fig. 10, cl). The other (Plate 80, fig. 10, c) is small, deeply 
pigmented, and runs along a groove in the heart to open into a circular canal at the 
base of the lantern. From the under aspect of this circular canal the five radial 
ambulacral vessels (Plate 80, fig. 10, i) take their origin, and, after passing under the 
rotulae and over the interalveolar muscles, they run along the inner surface of the 
ambulacral plates. The first series of pedicels (Plate 80, fig. 10, ;) projects through 
the oral floor midway between the oral aperture and the margin of the shell. 
Their respective ampullae (Plate 80, fig. 10 , hi) are long delicate tapering tubes which 
project upwards and outwards between the radial canals and the alveoli. The next 
four or five pairs of pedicels also pierce the oral floor. Their ampullae are small 
rounded sacs, whereas the ampullae of the first series of feet external to the auricles are 
slightly flattened and sometimes deeply constricted, whilst those beyond are in the 
form of flattened sacs lying at right angles to the radial canals (Plate 80, fig. 13, a). 
Immediately within the oral margin of the shell, and alternating with the inner row 
of pedicels, are the five pairs of “tree-like organs.” There is no evidence of the 
existence of these structures within the shell until a membrane (Plate 80, fig. 10, m), 
which extends from the apex of each tooth to the oral margin of the interambulacra 1 
plates and sides of the alveoli, is divided. If instead of dividing this membrane a fine 
glass canula be forced through it and a solution of Berlin blue introduced into the 
space between the membrane and the alveoli of the lantern, the fluid slowly diffuses 
upwards into, and greatly distends, the vesicles around the apices of the teeth (Plate 80, 
fig. 10, n). The fluid reaches these vesicles partly by passing directly upwards 
