538 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



longitudinal slit on the dorsal surface, two-thirds up the 

 joint from the posterior end. The opening is guarded by 

 two fringes of setae, an upper fringe of simple styliform 

 hairs springing from the inner edge and passing 

 diagonally forwards across the opening, and a lower 

 fringe of large, somewhat fusiform, densely plumose 

 hairs springing from the outer edge of the opening, and 

 directed at right angles to the upper fringe. 



The otocyst itself is attached by its anterior end, 

 underneath the opening to the exterior, and extends 

 backwards for about one-half the length of the entire 

 joint. It is a simple gourd-shaped sac with chitinous 

 unfolded walls, which are often — with the exception of 

 a small portion of the dorsal surface — lightly calcified. 

 The stalk of the gourd is at the extreme posterior end of 

 the sac, and is turned towards the mid-line, so the sac 

 appears to lie on its side. This small caecum has been 

 unfortunately named the " cochlea," in reference to an 

 imaginary resemblance to that structure of the vertebrate 

 ear. The interior of the "cochlea" is always minutely 

 spinose. A slight carina runs from the " cochlea " 

 round the outer side of the sac, and a shallow groove from 

 the same point along the ventral side contains the otic 

 branch, of the antennulary nerve. 



The interior of the sac is not complicated by any 

 irregularities, but follows precisely the lines of the 

 outside. The sensory region is to be found on the floor, 

 and the special "auditory" setae are roughly arranged 

 in two or three rows diverging from the " cochlea " ; the 

 greater number of setae spring from the slight ridge of 

 the otic nerve on the outer side of the sac, while the 

 remainder are placed in an irregular row on the inner 

 side supplied by a smaller nerve branch. They are of 

 one kind only — small, straight and plumose, miniatures 



