306 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
are the spheeridia (Pl. IT., fig. 18). They consist essen- 
tially of a hard, calcareous sphere, which differs from 
the ordinary skeletal structures in being concentrically 
laminated (sph.) The typical spheeridium is described as 
resting upon a short calcareous stalk, but this does not 
appear to exist in the present species, its place being taken 
by a cushion of nerve fibres. The typical spheridium has 
also a circular nerve ganglion and a capsular muscle 
around its base, the ectoderm covering which is specially 
thickened and bears long cilia. The spheridia are most 
probably sensory organs, and have been variously supposed 
to have an auditory or gustatory function, or, perhaps, 
to be concerned in the orientation of the test. 
At the peripheral edge of the peristome five pairs of 
hollow, branched appendages project freely outwards, and 
form the external gills (Pl. L., fig. 7, ex.g.). One pair lies 
in each interradius, and the peristomial edge of the test is 
slightly notched for their reception. Their contractile 
walls consist of the ectoderm, of which the cells are deep 
and bear long cilia; a connective tissue layer containing 
C-shaped spicules and cribriform plates (Pi. I., figs. 5 and 
6), and an inner epithelium continuous with that which 
lines the body cavity. The cavities of the gills open into 
the peri-cesophageal sinus. 
ALIMENTARY CANAL AND ‘‘ ARISTOTLE’S LANTERN.” 
The mouth is situated in the centre of the peristome, 
and the five pointed teeth project from its rounded opening. 
It opens directly into the pharynx (Pl. L, fig. 12, ph.), 
which passes upwards through the centre of the masticatory 
apparatus, or “‘ Aristotle’s lantern.” The latter consists 
of twenty principal skeletal pieces, which together form a 
pyramid with five rounded sides, and a complex system of 
muscles and ligaments (Pl. III., fig. 23; Pl. V., fig. 37). 
