Cuar. XVII STRUCTURE OF THE BLADDER. 401 
to a certain extent graduate into one another. Those 
situated round the anterior margin of the valve (upper 
margin in fig. 19) are very numerous and crowded 
together ; they consist of an oblong head on a long 
pedicel. The pedicel itself is formed of an elongated 
cell, surmounted by a short one. The glands towards 
the free posterior margin ,are much larger, few in 
number, and almost spherical, having short footstalks ; 
the head is formed by the confluence of two cells, the 
lower one answering to the short upper cell of the 
pedicel of the oblong glands. The glands of the 
third kind have transversely elongated heads, and are 
seated on very short footstalks; so that they stand 
parallel and close to the surface of the valve; they 
may be called the two-armed glands. The cells form- 
ing all these glands contain a nucleus, and are lined 
by a thin layer of more or less granular protoplasm, 
the primordial utricle of Mohl. They are filled with 
fluid, which must hold much matter in solution, 
judging from the quantity coagulated after they have 
been long immersed in alcohol or ether. The depres- 
sion in which the valve lies is also lined with innu- 
merable glands; those at the sides having oblong 
heads and elongated pedicels, exactly like the glands 
on the adjoining parts of the valve. 
The collar (called the peristome by Cohn) is evi- 
dently formed, like the valve, by an inward projection 
of the walls of the bladder. The cells composing the 
outer surface, or that facing the valve, have rather 
thick walls, are of a brownish colour, minute, very 
numerous, and elongated ; the lower ones being divided 
into two by vertical partitions. The whole presents a 
complex and elegant appearance. The cells forming 
the inner surface are continuous with those over the 
whole inner surface of the bladder. The space be- 
