304 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
the possession of a long stalk and a head composed of 
three blades. | 
The largest form is the tridactyle (Pl. II., fig. 138). A 
calcareous rod gives rigidity to about two-thirds of the 
length of the stalk, the remainder of which is flexible, and 
capable of movement in every direction. ‘The blades 
composing the head are long and slender, and meet only 
at their bases and apices when apposed to each other. 
Ligamentous fibres bind the blades together; and they 
are moved upon each other by three powerful adductor 
muscles (Pl. IL., figs. 18, 16, 19, and 21, ad.m.) whichvun 
between their adjacent sides. Nerve fibres pass upwards 
in bundles from the stalk, and are distributed to the 
sensory cells of the epithelium which clothes the inner 
suriace of each blade (PI. II., fig. 16, nf). 
The second form is the ophiocephalous or snake headed 
(Pl. IL., fig. 15). In it the calcareous rod which supports 
the stalk is relatively shorter than that of the tridactyle 
form. The three blades forming the head are scoop-like, 
with blunt apices, and each one has a sort of handle at its 
base. Figure 19 represents the head, with blades open, of 
a very similar but much smaller form which occurs in 
numbers on the peristome. 
The third form is the trifoliate, in which the blades of 
the head are broad and leaf-like, and meet only along their 
lateral edges when closed (Pl. I1., fig. 17). The ectoderm 
forms a thick cushion on the inner face of each. 
The fourth form is the gemmiform or glandular (Pl. IL, 
fig. 14). In it the calcareous rod of the stalk extends 
to the head, which is consequently much less capable 
of independent movement. The apex of the calcareous 
skeleton of each blade is directed inwards and forms a 
strong tooth, to which are added several smaller teeth. 
Each blade contains a glandular sac (Pl. I1., figs. 21 
