522 
PROFESSOR W. THOMSON ON THE EMBRYOGENY OF ANTEDON 
turbid with oil-cells and granules. At this stage distinct molecular motion may he 
observed in the central portion, and a granular semifluid mass escapes if the larva be 
ruptured by pressure. The surface is dotted over with the wider ends of large pyriform 
lemon-coloured oil-cells immersed perpendicularly in the sarcode. Between these oil- 
cells the sarcode is nearly transparent, containing merely a few scattered granules. The 
ciliated bands project slightly above the general surface. They are greyish and granular, 
and appear to be rather more consistent than the surface of the sarcode, which rises 
up to them, sinking somewhat in the interspaces. The cilia are very long; they do 
not vibrate with the regular rhythmical lash of ordinary cilia, but seem to move 
independently, their motion regulating the rapidity and direction of the movements of 
the animal in the water. There is a large tuft of still longer cilia in perpetual vibratile 
motion at the narrower (posterior) extremity of the body. At first the pseudembryo 
is simply barrel-shaped, and regularly hooped by the four parallel transverse ciliated 
bands. Sometimes, while yet within the vitelline sac, but at all events within a few 
hours after its rupture, the body becomes slightly curved, somewhat like a kidney bean ; 
and on the concave surface, the third band from the anterior extremity arches forwards 
towards the second band ; and in the wider space thus left at this point between the 
third and fourth bands, a large pyriform inversion of the superficial sarcode-layer takes 
place (Plate XXIV. fig. 7). 
This inversion is narrower anteriorly, becoming wider and deeper towards the poste- 
rior extremity. Its margins are richly ciliated. Simultaneously with the appearance 
of this depression, a small round aperture may be observed immediately behind it, sepa- 
rated from it by the fourth ciliated band, and close to the posterior tuft of cilia. This 
aperture is surrounded by a ring of darker granular tissue, and the outline of a short 
arched canal may be detected passing under the fourth ciliated band and uniting the 
deep posterior extremity of the larger aperture, which thus becomes irregularly funnel- 
shaped, with the smaller circular opening. 
The large ciliated key-hole-like inversion of the sarcode is undoubtedly the pseudo- 
stome ; and resembles closely in form and position the same organ in other echinoderm 
pseudembryos. The loop-like canal beneath the posterior ciliated band is the extremely 
rudimentary pseudocele, and the round aperture is the pseudoproct. The pseud- 
embryo swims with either extremity in advance indifferently ; the anterior and posterior 
extremities are therefore only defined at this stage by the relative positions of the mouth 
and anus. It swims rapidly with a peculiar swinging semi-rotatory motion. The oral 
surface is turned downwards in a state of rest. The pseudembryo sometimes remains 
for several days, increasing in size till it becomes from 1*5 millim. to 2 millims. in 
length, without undergoing any further change. In other cases indications of the areo- 
lated calcareous plates of the Echinoderm appear within a few hours of the rupture of 
the vitelline sac. 
Usually not until the pseudembryo has assumed its mature and perfect form, but 
sometimes much earlier, several minute calcareous spicula make their appearance beneath 
