544 PEOFESSOE W. THOMSON ON THE EMBEYOGENY OF ANTEDON EOSACEHS. 
ring, ten in number, and laid up in a state of complete expansion in pairs 
against the inner surfaces of the oral valves f; d, first pair of extensile radial 
tentacles ; e, azygous radial extensile tentacle leading out the growing arm to 
its bifurcation, and giving off pairs of tentacles of the same series from its base. 
X 40 linear. 
PLATE XXVII. 
Fig. 1. Pentacrinoid larva immediately before the expansion of the ventral disk: a, 
centro-dorsal plate ; b, series of basal plates ; c, first radial plates ; d, second 
radial joint; e, third radial ; f, first brachial joint; g, anal plate; h, stem- 
joint ; k, cribriform plate supporting the disk of attachment ; l, granular vis- 
ceral mass ; to, csecal process passing from the stomach towards the papilla 
which indicates the position subsequently occupied by the anal tube ; n, oral 
valve and plate. X 40 linear, slightly reduced. 
Fig. 2. An example in a somewhat earlier stage, expanded, and showing the arrangement 
of the non-extensile tentacles in connexion with the oral vascular ring, X 40 
linear, considerably reduced. 
Fig. 3. End of an extending arm further enlarged : a, 5, and c, first, second, and third 
radial joints ; d, superficial spicules and small cribriform plates of the peri- 
somatic system ; e, lenticular “ gland’”? ; f, radial vessel passing out on the arm 
to terminate in the azygous extensile tentacle A, after giving off the second 
paii’ of extensile tentacles k, Jc ; g, leaf and pair of tentacles of the non-extensile 
tentacular system. X 40 linear. 
Fig. 4. Pseudembryo uncompressed and observed by reflected light : a , pseudostome ; 
b, pseudoproct ; c, c, c, c, ciliated bands. X 40 linear. 
All the figures, except Plate XXVII. fig. 4, have been drawn from specimens under 
slight pressure, and with a special view to the details of internal structure. The contour 
has been thus in some cases to a certain extent lost, and the figures, especially those of 
the pseudembryo, must be understood to represent individuals slightly flattened. 
