ETTPAGTTRTTS. 539 



in all but the base, of the fringing setae already 

 described (fig. 36). Each hair is a hollow tube attached by 

 a membranous base to a knob-shaped podium, in which 

 the single nerve fibre supplying the structure terminates. 

 Thus the hair is only capable of extensive lateral move- 

 ment at the membranous base in direct contact with the 

 nerve element. 



Many sand particles, of all sizes small enough to 

 enter by the orifice, are found free in the lumen of the 

 sac or adhering to the setae. The nerve supplying the 

 sensory region impinges on the sac at its posterior end 

 on the ventral surface, and at once divides into a more 

 ventral broad portion which curves nearly round the 

 outer side of the sac, and a smaller branch passing to the 

 inner side.* It is unnecessary to deal with the functions 

 of the sac in detail. Prentisst gives an exhaustive 

 account of the work of earlier observers and supplements 

 it with his own experiments. A feeling for the 

 picturesque in the earlier naturalists, aided by incon- 

 clusive experimental work, led them to assign a sense of 

 hearing to Crustaceans only differing in degree from our 

 own ; but, after a period in which both opinions were 

 held, the general view now is that the otocysts are almost 

 exclusively static in function, and are only concerned in 

 the orientation of the animal. 



Olfactory and Tactile Setae. 



Other sensory setae of a simple kind are found in 

 most parts of the body. They consist of a hollow shaft, 

 which communicates with a single nerve fibre. On the 



* An inaccurate account of the otocyst of this species (under 

 the name P. streblonyx) by Farre, may be found in Phil. Trans. Lon. 

 1843. 



t Bull. Mus. Compar. Zool, Harvard, Vol. XXXVI., No. 7, 

 p. 168, 1901. 



