ON THE LOCOMOTOR SYSTEM OF ECHINODERMATA. 
833 
the form of a large oval sac (Plate 80, fig. 11, d). The size and form of the vesicles 
are largely determined by the amount of fluid in the pedicels, the vesicles diminishing 
when the feet are protruded, and enlarging when they are retracted. 
The first series of ampullae (Plate 80, fig. 11, e ) are small semidunar sacs, which lie 
in close contact with each other opposite the origin of the polian vesicles, and between 
the ambulacral canal and the circular vessel. The other ampullae (Plate 80, fig. 11,/’) 
are spherical in form. In none of the injected specimens was there any evidence of a 
communication between the ambulacral vessels and the body cavity, or between the 
ambulacral 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 watery solution could be easily forced 
through the sinus into the circular blood-vessel (Plate 80, fig. 11, h), and from the 
circular vessel into the radial blood-vessels. But when the canula was introduced into 
the proximal end of the sinus, the solution rapidly rushed along the sinus and escaped 
freely through the madreporic plate—proving that the blood-vessels of Solaster com¬ 
municate far more freely with the exterior than do the water vessels. 
5. Uraster and Astropecten. —The ambulacral system of the common Star-fish 
(Uraster rubens ) differs from that of the Sun-star ( Solaster ) only in having no polian 
vesicles. Astropecten ( A. aurantiaca), on the other hand, has polian vesicles ; but in 
it the pedicels have departed from the typical form. In Holothuria and Echinus the 
feet terminate in well-marked sucking discs (Plate 79, fig. 6), which have their margins 
frequently strengthened by a deposit of calcareous matter. All the pedicels of Solaster 
and Uraster, with the exception of a few at the tip of each arm, are also provided with 
suckers. Those at the tips of the arms are long and pointed, and when the Star-fish 
is moving they project forwards in the direction of advance, and appear to act the part 
of tentacles. In Astropecten, however, the feet are short and conical, and instead of 
ending in suckers they terminate in rounded points (Plate 79, fig. 7). But although 
suckers are absent, Astropecten is able slowly to ascend a vertical surface. We have 
repeatedly observed slight inversions of one side of the pedicels near their pointed tips 
when this Star-fish slowly ascended from the bottom of a glass aquarium ; hence we are 
inclined to believe that Astropecten has the power of converting a portion of the side 
of its pedicel into an imperfect sucker (Plate 79, fig. 8, a). 
6. Ophiura. —In the Brittle and Sand-stars, the ambulacral feet are morphologically 
similar to those of Astropecten, though shorter and more slender. Those near the disc 
further differ from the pedicels of Astropecten in being more pointed (Plate 79, fig. 9). 
The pedicels beyond the disc gradually diminish in size, and at the ends of the arms 
they are scarcely visible. All the feet are devoid of suckers, and no attempt is ever 
made to form even a temporary imperfect sucker by slightly inverting a portion of the 
side of the foot. 
5 P 
MDCCCLXXXI. 
