312 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
Pl. IV., fig. 29; Pl. V., fig. 37, am.tf.), which project, 
like the leaves of a book, into the interior of the test. 
Each ampulla communicates with the cavity of a tube-foot 
(l.t.f.) through two canals (Pl. IV., fig. 29, c.t.f.), which 
traverse a pair of pores of an ambulacral plate. The tube- 
feet are tubular organs which occur on the piates of the 
ambulacra only. They are exceedingly mobile, and can 
be extended far beyond the tips of the spines. Their 
free, imperforate ends take the form of circular sucker 
discs, by means of which the animal can attach itself with 
creat firmness to the surface on which it rests. 
Examination of a thin section of one of these organs 
(Pl. III., fig. 27) reveals the following structures :—(1) an 
external ciliated epithelium (ect.) continuous with that 
of the general surface of the body; (2) a delicate plexus 
of nerve fibres and ganglion cells; (3) a layer of connec- 
tive tissue (c.t.l.) in which C-shaped calcareous spicules 
(Pl. I., fig. 5) occur; (4) a layer of longitudinally disposed 
muscular fibres (m.f.l.); (5) a ciliated epithelium lning 
the cavity of the organ (ep.). The terminal sucker is 
supported by an elegant, ring-like calcareous plate, com- 
posed of six or seven segments which, like the C-shaped 
spicules, lies embedded in the connective tissue layer. 
The sucker discs of the buccal tube-feet are, in like manner, 
supported by a ring-like plate, but in conformity with the 
shape of the discs, they are oval, and composed of four 
or five segments only. ‘The epithelial cells which cover 
the external surface of the sucker discs are much longer 
than those of the tubular portion of the tube-feet, and are 
not ciliated. 
The minute structure of the ampulle is similar to that 
of the tube-feet. In a section there may be seen from 
without inwards :—(1) the ciliated epithelium lining the 
body cavity; (2) a layer of connective tissue, containing 
