184 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
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and a lower sacculus (fig. 183), the two connected by a narrow sac- 
culo-utricular canal. The sensory area becomes divided between. 
the two, but the endolymph duct is connected with the sacculus alone. 
The anterior, posterior and lateral walls of the utriculus now produce 
flattened outgrowths, the lateral in the horizontal, the others in vertical 
planes, and parts of the sensory areas extend into each. Next, the 
walls of these diverticula become pinched together so that each pocket 
Fic. 184.—Diagram of the membranous labyrinth of a vertebrate, the sensory areas 
dotted. ac, anterior semicircular canal; ap, ampulle; ca, criste acustice in the ampulle; 
de, ductus endolymphaticus; hc, horizontal (external) canal; 1, lagena; ml, mn, ms, mu, 
macule of lagena (neglecta, sacculi and utriculi); pc, posterior semicircular canal; s, 
sacculus; se, saccus endolymphaticus; suc, sacculo-utricuar canal; u, utriculus. 
is converted into a tube or canal, open at either end into the utriculus, 
and hence approximately semicircular in outline. In one end of each 
of these semicircular canals there is a patch of sensory epithelium 
and the wall expands around this into an ampulla, the ampull of the 
anterior and external canals being side by side, that of the posterior 
canal at its lower end. 
In the lower ichthyopsida there is little differentiation in the sac- 
culus, but in the higher a pocket, the lagena, is given off from its poster- 
ior side, a portion of the sensory epithelium extending into it. With in- 
