870 
MR. G. J. ROMANES AND PROFESSOR J. C. EWART 
sand and the remains of food particles. When the tentacles are being protruded 
more water is taken in at the cloaca than escapes ; on the other hand, retraction of 
the tentacles is preceded by an escape of a large stream of water. 
In Echinus two tubes spring from the under surface of the madreporic plate. The 
one is dilated at its origin so as to include the greater portion of the plate, and ends 
in the so-called heart; the other 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 ambulacra! vessels take their origin. 
Immediately within the oral margin of the shell and alternating with the inner row of 
pedicels, are the five pair of “tree-like organs.” If a fine glass canula be forced 
through the membrane which extends from the apex of each tooth to the oral margin 
of the inter-ambulacral plates and sides of the alveoli, coloured fluids may be injected 
into the space between the membrane and the alveoli of the lantern; the fluid then 
slowly diffuses upwards into the vesicles around the apices of the teeth. It reaches 
these vesicles partly by passing directly upwards external to the alveoli, and partly 
by passing into the cavities of the alveoli and ascending through the circular sinus. 
In Spatangus the ambulacra! circum-oral canal lias no polian vesicles or sinuses 
developed in connexion with it. Some of the pedicels have suckers, others are conical 
and devoid of them, while others again are flattened at their tips, and sometimes split 
up into segments. 
If one of the arms of Solaster papposci is divided transversely and a coloured fluid 
is introduced into the open end of the radial canal, the ampullae and pedicels of the 
injected arm are at once distended. The fluid next penetrates the circular canal, 
polian vesicles, ampullae and pedicels of the other arms; but unless considerable 
pressure be kept up for some time, none of the solution enters the madreporic canal. 
If, however, the pressure is maintained for several hours with a column of fluid 2 feet 
high, the fluid ascends through the stone canal and diffuses slowly through the 
madreporic plate. When a thin slice is then shaved off the plate, the fluid is 
observed escaping from a small circumscribed area situated between the centre and 
the margin of the plate, and corresponding in size and position with the termination 
of the stone canal on the inner surface. The stone canal gradually increasing in 
diameter as it passes inwards from the madreporic plate, runs obliquely over its 
accompanying sinus, till it finally hooks round this sinus to open into the circular canal. 
Springing from this canal and opposite to each inter-radial space (with the exception 
of the space occupied by the stone canal) is a polian vesicle. The size and form of 
these vesicles are largely determined by the amount of fluid in the pedicels. In none 
of the injected specimens was there any evidence of a communication between the 
ambulacra! vessels and the body cavity, or between the ambulacra! and the blood 
(neural) vessels. There was, however, abundant evidence of communication between 
the latter and the exterior. When a canula was introduced into the outer end of the 
sinus, a coloured solution could be easily forced through the sinus into the circular 
