CHAP. XVII. STRUCTURE OF THE BLADDER. 401 



to a certain extent graduate into one another. Thoso 

 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- 



