832 
MR. G. J. ROMANES AND PROFESSOR J. C. EWART 
external to tlie alveoli, and partly by passing into the cavities of the alveoli and 
ascending through the circular sinus. When a solution of coloured gelatine, or what 
is better, plaster of Paris, is injected into the space above the tentacles, or into the 
vesicle around the soft apex of one of the teeth, a cast is readily obtained of the 
circular sinus and of the spaces communicating with it. A vertical section of the 
lantern of an urchin thus injected shows the wedge-shaped circular sinus (Plate 80, 
fig. 10, e) lying between the radius and rotula, communicating above with the vesicle 
around the tip of the tooth (Plate 80, fig. 10, n) and below with the cavity of the 
alveolus (Plate 80, fig. 10, p), through which the tooth passes, and by means of the 
latter cavity communicating indirectly with the large space lying above the “ tree¬ 
like organs.” 
3. Spatangus. —In Spatangus ( S. purpureus ) the ambulacra! circum-oral canal 
has no polian vesicles or sinuses developed in connexion with it. The ampullse imme¬ 
diately around the oral aperture are tubular, and often measure a quarter of an inch 
in length. Those beyond on the under surface and round the equator are also tubular, 
but they are small and few in number. This is true of all the ampullae of the anterior 
radial canal. The ampullse which project inwards from the dorsal portions of the four 
remaining radial canals are, as in Echinus , transversely flattened sacs. Some of the 
pedicels have suckers, others are conical and devoid of suckers, while others again are 
flattened at their tips, or flattened and split up into segments. 
4. Sol aster.— When one of the arms of Solas ter papposa is divided transversely an 
inch from the disc, and a coloured solution is introduced into the proximal portion of 
the radial canal, the ampullse and pedicels at the base of the arm injected are at once 
distended. The solution next penetrates the circular canal, polian vesicles, ampulla?, 
and pedicels of the other arms; but unless a considerable pressure be continued for some 
time, none of the solution enters the madreporic canal. In one specimen, when the 
injection had been continued for five hours with the pressure bottle raised two feet 
above the level of the Star-fish, the solution had ascended two-thirds of the entire 
length of the stone canal, and two hours later it began to diffuse slowly through the 
madreporic plate. When a thin slice was removed from the surface of the plate, the 
solution was obserVed escaping from a small circumscribed area (Plate 80, fig. 12, a) 
situated between the centre and margin of the plate—an area corresponding in size and 
position with the termination of the stone canal on the inner surface. 
Starting from the inner aspect of the madreporic plate the stone canal gradually 
increases in diameter, and passes obliquely over the accompanying sinus till it finally 
hooks round the sinus to open into the circular canal (Plate 80, fig. 11, a). Springing 
from this canal, which occupies a sinuous groove on the dorsal aspect of the inner 
ambulacra! ossicles, and opposite each interradial space (with the exception of the space 
occupied by the stone canal) is a polian vesicle (Plate 80, fig. 11, c). Each vesicle 
consists of a tubular stem measuring from two lines to a quarter of an inch in length, 
and of a dilated portion which may be exceedingly small (Plate SO, fig. 11, i), or takes 
